get rid of some rep you feel was ill-gotten (like an exceedingly simple answer that was upvoted into oblivion)
generally spend some rep
And you don't personally have a question that you need answered. What questions in the Stack Overflow ecosystem would benefit especially from some bounty love? How do I invest a bounty for the benefit of the question & answer base?
How can I identify those questions?
How can I find some outside the tags I frequent?
Are there characteristics that define those questions?
Do you know any specific questions that could benefit from a bounty?
One way to find good questions without answers is going to the Unanswered Questions tab - it's sorted by votes count so you will clearly see questions with lots of upvotes and without answers. (Without upvoted answers, to be totally accurate)
Just sift through this list and you ought to encounter some that can use a fresh bounty.
Being heavily upvoted usually means it's good, as many people think it's good and having no answer usually means those questions are just too complicated - more the reason to place a bounty.