You use "I guess" to show that you have changed your mind when someone is trying to convince you of something. For example:

A: Why don't you stay and have one more drink?

B: OK, I guess I can have one more.

In this example, speaker B makes it seem that it was speaker A's idea to stay and have another drink.

"I guess" is a phrase which shows that you're making a decision that's different from what you originally were planning on doing. If you simply say "I will ___", it sounds as if you wanted to do it from the start:

I'll stick around and have a few more drinks.