CPM vs CPC: What are the advantages to choosing either bid type?

Okay so this might be a bit of a newbie question, but I couldn't find anything on it in the wiki

I wanted to ask: in what scenario is it better to use CPM? Because I have been doing only CPC campaigns so far and I'm curious as to what the benefit would be to pay per 1000 impressions instead of pay per click.

I see many different affiliates using both bid types, but nobody ever explains why...

I hope some members might be able to give their insight on this thanks.

* This article was originally published here