Some management, some sales people, some consultants (where there can be a "don't go work for a client you have been recently placed at, for N months after leaving" clause). Mostly, I have seen it done through gardening leave, though.

But, as I understand it (but, then, I have NO legal training), if the no-compete is baked into your employment contract, it is null and void after your employment is over. To actually be valid, it needs to be a separate contract. I would double- and triple-check that before relying on it, though.