Something which I only realised by watching a London Underground driver who was vlogging for a while (before he was asked to stop) is that, yes, the frequency of the trains is the main metric for customers.

However, once things go off schedule it can have pretty acute consequences for drivers and operational staff. The timetables are built around having drivers at specific places at specific times who will have required breaks at certain times and a maximum allowable working time. The rolling stock itself needs to be where it's needed as well, it's easy for it to get bunched up behind a track circuit failure etc.