Why 60 minutes
An hour is the default unit of professional time, the standard slow-cooker check-in, and long enough that you should not try to hold it in your head. Set it, walk away, and let the alarm do the remembering.
It will not drift while you do something else
The countdown is worked out from a fixed end time rather than counted down frame by frame, so putting the tab in the background, locking your phone, or switching apps does not cost you any of the 60 minutes. Browsers throttle background JavaScript to save battery, and that is exactly what makes most browser timers lose time. Press start, go and do the thing you are timing.