It modifies some stats, including one that is very difficult to modify otherwise, and potentially adds random affects with each cast. Additionally, it allows spells to bypass abilities that would otherwise block or deflect them.

If I'm being vague, it's because:

A) I don't have code access and only know what I've heard and seen using it on myself and other IMMs, which does not necessarily translate into mort vs mort.
B) Few people have tried Entropy, so this isn't really common knowledge yet.