Early game the main use is for underground to allow different belt lines to cross. Take a look at the standard green circuit setup 3 copper to wire assemblers direct feed into 2 iron + wire to green. It uses underground to move iron past the output.

For better belts upgraded when you need the speed. When your yellow can't push enough product to feed your machines. Also better underground belts can go father between ends. 4, 6, 8 empty tiles for yellow, red, blue.