A Basic Question : Why regeneration required in Creo?

What are these other "CAD tools" of which you speak? Doing a cursory search I find regeneration used in two of the prime competitors to Creo, NX6 and SolidWorks. Perhaps you are comparing a non-feature based modeling program to this one?

Here's an example of a situation where regeneration is welcome: You're working on a horrible model that has upwards of 800 features in it. You want to make some small changes to three or four dimensions, nothing major, just slight adjustments. Presumably, if regeneration didn't exist, your model would have to update automatically each time you made a change to one of the dimensions. So, you'd be forced to sit through a series of lengthy recalculations and updates. And, if you weren't careful in the order of the changes you made, you could also suffer the nightmare of a failure. It's much better to be able to modify all of the dimensions necessary, then just sit through one regeneration. This isn't a hypothetical case, either, I've had run-ins with this type of thing before.

In "olden times", as I recall, the models we created were "dumb" models, in the sense that you had the final geometry, but didn't have all the intermediate stages available to you. If you wanted to "fix" a feature, you couldn't roll back the model and change the operations that created it. You had to patch up your model in some clunky manner and then do some more work to get the correct end result. Good stuff for concocting horror stories, but not for getting things done efficiently.