When I was exploring on finding the reason for having FAST Search for SharePoint, I came across this article at Microsoft Blog and I would like to share the same with you all.

TOP 10 Reasons for considering FAST Search on SharePoint 
Content Processing Pipeline 
For people familiar with the FAST Enterprise Search Platform (ESP), the good news is that the most valued capabilities of ESP have been brought into FAST Search for SharePoint 2010 and made easier to access and deploy through tight integration with the SharePoint management and development tools.  The open framework in FAST ESP for creating custom content processing pipelines is a good example. Since it was first introduced in version 3 way back in 2002, FAST customers and partners have leveraged advanced content processing and advanced linguistic features to create a wide variety of novel search applications. This highly valued aspect of the FAST ESP will be available in FAST Search for SharePoint and has been architected and enhanced to take advantage of the SharePoint management interfaces and development tools like PowerShell.  
Meta-data Extraction 
Meta-data is used in search for faceted refinement, relevancy tuning, targeted queries (e.g. search only the authors field), and other general techniques designed to improve findability. The problem is that unstructured documents are often devoid of useful meta-data. The ability to automatically extract meta-data to create useful structure on otherwise unstructured documents is a feature of FAST ESP that will also available in FAST Search for SharePoint 2010. Importantly, FAST Search for SharePoint 2010 takes advantage of simple administrative tools and the concept of “managed properties” in SharePoint to support adding custom meta-data extractors very quickly.  
Structured Data Search 
Structured data search is possible with both search options in SharePoint 2010, but FAST Search for SharePoint 2010 adds an extra level of sophistication for searching data that contains numbers, dates, and other encoded and structured information. To start, the full FAST Query Language (FQL) is available to application developers who want the richness and expressiveness that FQL provides. This includes support for numeric and date data types, formula-based query operations, term weighting with the XRANK operator, and much more. Also, integration with the new Microsoft Business Data Connectivity services in 2010 means that ingesting structured data from external Line of Business applications is much easier in FAST Search for SharePoint.

  “Deep” Refinement (Faceted Search) 
Previously only available in SharePoint search through 3rd party add-ons, faceted search, called “refiners” in the default search interface (SharePoint Search Center), is now native in the out-of-box SharePoint 2010 search. FAST Search for SharePoint adds to this the ability to deliver faceted search across results sets of any size while retaining precise counts on the refinement facets. This is critical for research and analysis applications where precise counts on facets are important decision making criteria. (You can see examples of deep refiners on FAST ESP powered sites like scirus.com and dell.com.) 
Visual Search (Document Thumbnails and Previews) 
Visual document thumbnails and previewer Web Parts will be out-of-the-box with FAST Search for SharePoint 2010 to help users more quickly judge what is relevant in a search result list. This includes a graphical previewer for PowerPoint presentations based on Microsoft Silverlight that allows users to quickly find the “one slide” of interest without having to open up the entire presentation. 
Advanced linguistics 
The quality of search against text data is highly dependent on the ability to apply the right language-specific processing techniques. FAST Search for SharePoint 2010 builds on the FAST ESP heritage and Microsoft tools to include advanced language processing (linguistics) for dozens of languages, including optimized processing for Chinese/Japanese/Korean. 
Visual best bets 
SharePoint already supports the concept of search Best Bets – managed results delivered with the search for specific queries. FAST Search for SharePoint adds to this the ability to render visual best bests in the form of images and even videos. Management of search best bets, both standard and visual, is through the standard SharePoint administrative console.

Best-in-class development platform 
FAST Search for SharePoint 2010 builds on the comprehensive development framework of SharePoint 2010. The customization options range from configuring out-of-the-box search behavior (best bets) and user interface controls (Web Parts), to extending existing functionality using public Web Part code and SharePoint Designer, to creating brand new components and functionality with the available APIs. For FAST ESP aficionados, no compromises have been made in the area of extensibility with FAST Search for SharePoint, but many of the customizations in ESP are now much easier to do.  
Custom search experiences (per user/profile) 
FAST Search for SharePoint 2010 includes the same level of relevancy tuning available to FAST ESP. It will be possible, as it is in ESP, to create custom relevancy models tuned to differences in content sources, application needs, and user contexts. User context simply means that different users can have different search “contexts” that enable experiences optimized for their specific business needs. User context can be used to set the search sources, relevance rank profile, linguistic processing features, and other search features by user or user group. In an enterprise search setting, this means that a Sales Director does not have to see the exact same results as a Product Designer for a given query, even if they are searching the same sources. 
Extreme Scale and Performance 
Scale and performance of the out-of-the-box SharePoint 2010 Search has been dramatically improved – with proven scalability to 100 million documents and more. For FAST Search for SharePoint 2010, the exact same scale-out model that exists in FAST ESP has been preserved to enable extremes of content (e.g. number of documents to search), queries (e.g. the number of queries or query rate), or both. This means search solutions that can support billions of documents and thousands of queries per second.

Regards,

Kishore Kumar.N