What does it do that the site overlay in Google Analytics doesn't do?
There is a comparision on the website 3/4 the way down the page. As per their site:
Why Is Crazy Egg Different From Google Analytics?
Google Analytics doesn’t have a heat map, a confetti report or a scrollmap.
Google Analytics just has overlay... but wait, there’s even more.
Here is the problem with Google Analytics overlay:
Let’s say you have two links that go to the same place and one of them gets 15 clicks and the other one gets 25 clicks. In Crazy Egg you’ll see 15 and 25... whereas in Google Analytics you’ll see those two links with 40 clicks each.
Why?
Because Google Analytics uses your page-level data to track click maps, which is grossly inaccurate
For a long time Google Analytics has had this deficiency. In fact, that deficiency on the overlay is one of the main reasons we built Crazy Egg.