For commuter rail inside a city it's nice to know "the trains run every X minutes so I can always get one" - between cities it can be more clumped around commuter times.

You also run into trainset issues where you want to run 5 trains in one direction and 5 back in the evening, which will require 5 trainsets, but if you run them back and forth you could do more trips with less trainsets, but some would be running off-peak (and in the worst case, nearly empty, but getting into position).