I have apple snail eggs all over every piece of cover above the water table of my 25 acre lake.
This is a natural lake with a variety of fish, including bass, bluegill, crappie, and shellcracker (ie RES).

My question is, what role do the snails play in the ecosystem? Will they serve as a significant food source? Will they benefit the water quality?

I am not too educated on snails.

RES are famous for eating snails. They have special pharyngeal (OK, I don't know what it means either, probably greek for "on the tongue") teeth to allow them to crush the shells.

From personal aquarium experience I can testify that BG will also eat snails that they are able to suck out of the shells (kind of neat to watch). They devoured every snail we put in a 5 gallon tank over the course of a year; no snail lasted longer than a month. These were snails which looked very similar to the apple snail pics I found on-line, including a big one over an inch across with a "trap door" (he lasted the longest).

So your apple snails are subject to at least two sources of predation; that's probably why they are on top of the cover.

Anatomy time at PB fish college.

"tooth plates for pharyngeal (throat) teeth, or various flanges for the attachment of muscles."

"Pumpkinseeds and Redear sunfish have pharyngeal teeth for crushing invertebrates and can crush snail shells."

"The mouths shape is a good clue to what fish eat. The larger it is the bigger the prey it can consume. Fish have a sense of taste and may sample items to taste them before swallowing if they are not obvious prey items. Most freshwater fishes in Florida are omnivorous (eating both plant and animal matter). Some are primarily piscivorous (eating mostly other fish). The imported grass carp is one of the few large fishes that are primarily herbivorous (eating plants). Fish may or may not have teeth depending on the species. Fish like chain pickerel and gar have obvious canine-shaped teeth. Other fish have less obvious teeth, such as the cardiform teeth in catfish which feel like a roughened area at the front of the mouth, or vomerine teeth that are tiny patches of teeth, for example, in the roof of a striped bass' mouth. Grass carp and other minnows have pharyngeal teeth modified from their gill arches for grinding that are located in the throat."

I am certainly looking forward to seeing the RES working on these snails.