Couple basic questions about "thickened" epoxy.

I've thickened for two reasons so far. First, so that it will take the shape I want, such as the fillet around the cockpit coaming. Second, to get the color I want (again, the fillet around the coaming).

However, in other situations where neither of the above should matter, I've read that one should use "thickened epoxy". An example would be related to my last post - to glue the lip on the cockpit coaming.

So, is the thickening simply to reduce weight, or are there other reasons why you "need" to thicken it?

Rick