Why 120 minutes
Two hours is a slow roast, a feature film, and the length of most standardised exams. It is long enough that a timer is not a convenience but a necessity, and long enough that you should check the thing you are timing at least once before it ends.
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 120 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.