[Zope] ZClasses on top of my Python classes?

I've seen several doc tidbits suggesting one create ZClasses derived from their Python classes, sticking all the biz logic in the python classes, and the presentation in the ZClasses... Could someone provide some insight into why this is better than simply writing separate python classes for logic and presentation? (Although I have to admit hitting the "Restart Server" button every time I make a typo is getting old) Thanks! -Randy