Main purpose on doing performance tests in my mind

1> Help to find out the (Top) bottlenecks, so that we are able to focus on and make a plan to tune them one by one;
2> Help team/PM to make a decision on release by measuring the trend of performance, guarantee no degradation on each release. (I prefer to taking the throughput trend and overall performance data rather than individually response time as a degradation indicator)
3> Help dev to do the hot spot tuning, we will "agilely" and "aggressively" change our test strategy during tuning phase :)
check out My previous blog on performance test strategies:
http://joychester.blogspot.com/2009/04/performance-test-strategy-in-my-mind.html
4> Validate some "guessing" or "pilot" projects
5> Capacity planning, scalability testing（horizontally and vertically）
6> Bonus! Finding out functional or security bugs in application occasionally
。。。
Feel free to add yours：）