And before someone says it's similar to pleading not guilty when you are guilty, AFAIK, a plea bargain is different; It's an admission of guilt, not a declaration of guilt. Declaration != admission.
I got the impression it was because the state's attorney felt I would best learn my lesson with that outcome than the otherwise mandatory sentence for the crime with which I was originally charged — non-violent, mind you; it was a bounced check. Yes, really.
So in some cases, plea bargaining is a "way out" of otherwise too-harsh sentencing. Yes, the sentencing guidelines should be overhauled, but in today's legislative climate, the path of least resistance is simply to allow the prosecution the flexibility to charge a defendant with a lesser crime.
Just from a purely game theory perspective, as a defendant you can easily find yourself in a position where it's stupid to reject the bargain even if you're innocent.