For the thing you keep avoiding
Fifteen minutes is easy to agree to. You are not promising to finish, only to sit with the task until the chime. Often the hard part is the first few minutes, and by the time the timer ends you are past it. If you are not, you have spent fifteen minutes and can stop.
For a quick review or a small clean-up
It also suits work that is small by nature: going through your notes, clearing an inbox, tidying a draft. Three minutes off follows, and the next block waits for you to press start. If fifteen turns out to be too short for the task, the 25-minute timer is one click away.