For me its 2 fold.
1. During the sucker spawn itself, its a very match the hatch situation. Trout of all species will line up behind suckers actively spawning. Similar situation with steelhead runs. "Sucker spawn" is a pattern that belongs in the larger "egg pattern" category, which includes glo-bugs and others. Even the beads dirty pinners use! (had to get a cut in there, hehe, tongue planted firmly in cheek).
2. Year round, rainbows especially. They're just genetically programmed to eat eggs. I used to think it was just stockies, but try it on a wild bow stream in like August, you might be surprised! Any time of year, any place. I have used the term "cheat code", and it fits. Thousands of spinning rod guys with jars of Mike's are onto something, lol, but they often fish stocked streams with rainbows. Browns though, seem to only take egg patterns when natural eggs are on the menu. Brookies are in between. They'll take, but not necessarily better than other stuff, they just take anything. Egg patterns are primarily rainbow candy outside of an egg "hatch"!
I have always believed the venerable green weenie, while can at times represent any number of things, one of those things is to serve as an egg fly.