Underground downspout extensions - Are pop up emitters needed?
Pop up emitters are a good thing that do a few jobs.  They stop debris from falling back down into the pipe, but more importantly (in my area) they stop mosquitos from getting in and multiplying in the standing water.  If a grate was in place, you have open access to standing water.

They don't have a water-tight seal so they do not really stop water from back-flowing.  It's more for debris and to some it's a little "cleaner" looking than an open grate.