I'm personally using it to set a minimum time to devote time to a project. If I set a pomodoro for 25 minutes and it happens to go to 35 minutes, I'm OK.
I know, I know... Purists say to stop at 25 minutes and take a break. But if I'm in the flow, I gotta keep going. Sometimes, setting a 2 minute pomodoro to just open an app is what I need to kickstart my butt into gear.
If I finish a task in less than the 25 minutes, I can easily take a break for a minute to switch context/mind/mood and then start up another pomodoro.
It's all about experimenting and finding your groove. Some folks find a 25 minute pomodoro perfect. Others might want a slightly longer or shorter pomodoro. I wouldn't worry about being rigid and sticking to the suggested 25 minute duration.