Flies are important. Very very.

1. They are detrivores. This means that they eat dead organic matter. While they aren't decomposers, they aid the decomposition process by breaking down organic matter into small simpler forms so fungi and bacteria can complete the decomposition process. This means that flies help to recycle nutrients in the ecosystem
2. They're scavengers. This means that they're actively searching for food, they eat almost anything edible. Being scavengers, flies actually keep their environment clean as they will eat up the edible dirt.
3. They serve as food to larger organisms like frogs/toads, lizards, fish, etc.

See? Flies aren't that useless

Image source: Google