I think of suet as a type of insect substitute, since it's animal fat.
In winter, it provides fat for energy to help birds get through the cold nights. We offer suet in the summer, too, and we've learned it's important to buy the All-year variety so it doesn't melt or become rancid.
This suet feeder thwarts starlings
Sometimes we've had a problem with starlings. We watched as they not only chased away our native birds but also quickly ate up the suet we just bought.
When we discovered this other kind of feeder, it changed things immediately. The starlings aren't able to cling upside down, but this position seems pretty comfortable for chickadees and woodpeckers—the kinds of birds we're trying to provide for.
It seemed like the suet was being pushed away when birds pecked at it, though. I'm not sure if this would have been a problem, but we solved it by putting a flat stone between the suet and the roof of the feeder to keep it pressed close to the screen.