Compressor Wiring

[SCOTI]

My previous truck used just a 'bare bones' approach to air managment & it worked well. The system consisted of a small compressor, 2-gal tank, manual paddle valves, & the bags. The compressor was wired directly into my electrical system (compressor + lead to 1 pole of pressure switch; 2nd pole of pressure switch went directly to the POS batt terminal (30amp fused wire).

While that worked, it allowed the compressor to come on @ any time (not a problem then as I drove the truck daily). My dually will sit around a little more so I was thinking of utilizing a relay so the comp would only be 'hot' when the ignition is on.

I figured I would wire my duallys set-up the same as my previous truck but go through a relay vs. directly to the battery. But, when I researched compressor/relay wiring, most of the set-ups pictured seem to have both the compressor & pressure switch going through the relay.

I'm trying to figure out why both the compressor + & pressure switch would need to go through the relay and/or if my way would work too (knowing there's more than 1 way to utilize a relay).