Well 4lucky! it depends on your own interests. I mean, like if you are a member of some paid to click program and you want to promote your link for new referrals or want to promote your link to target users for something. You can use your banner imperssions for lots of things. You can even sell them for low cost, if I am not wrong.
If you know how and why to promote different programs then do gather some information about it so that you can fully utilize your banner impressions.

Best of Luck with It!