Thirty minutes is the everyday half-hour, and the uses are gloriously ordinary: an oven step, a power nap, a TV episode, a study block, a parking meter, a time-boxed chore. It is the general-purpose countdown for all the half-hour things that fill a day.
A round count in the editor repeats it — two rounds is an hour, split cleanly in half. For study or focus, a timer that adds a short break after each block usually works better than a flat repeat.
A 24 minute timer — about a sitcom with the ads cut out, or one long focus block. Repeats as many times as you set.
A 90 minute timer — a football match, a movie, or one full sleep cycle. Repeats if you need it to.