Because of the extensive use of plan parameterization, and the way that the Query Optimizer sniffs for parameters on each execution of a parameterized plan, SQL Server doesn’t always do the best job of choosing the right plan for a specific set of parameters.
The OPTIMIZE FOR hint enables you to tell the Query Optimizer what values you expect to see most commonly at runtime. Provided that the values you specify are the most common case, this can result in better performance for the majority of the queries, or at least those that match the case for which you optimized.
Before using any query hints, run a web search for the latest information on issues with query hints. Try searching on the keywords “SQL Server Query Hints” and look specifically for anything by Craig Freedman, who has written several great blog entries on some of the issues you can encounter when using query hints.