If flies do get into the coffin and lay there eggs in the coffin on the dead bodies then yes the maggots can eat the dead bodies which help to decompose the body.
Maggots will also eat dead bodies that are not in a coffin and if say a person dies outside and is not found then if another animal doesn't eat the body then maggots will indeed be laid on the dead body and eat the dead body.
If the body is not put into a coffin and is just out in the open and flies are around the flies will land on the dead body and then eventually the flies eggs turn into maggots and then eat the dead body.
Maggots on a dead body can also tell forensic scientists and detectives whether or not a body has been moved and how long the body has been dead as well.
If the dead body isn't taken to a morgue and put into a fridge to keep the body cool and away from flies then maggots can appear on a dead body within 24 hours of death.