Welcome! I am with you on the deep substrate and lots of plants. My Sumatra tank doesn't even have a filter, just a tiny pump to push things around. I do water changes simply because it makes sense to me. Living things put lots of chemicals into the water besides the big three (nitrate, nitrite, ammonia), chemicals that plants use to inhibit other plants; chemicals that animals use to communicate with each other (there's a name for all those but I can't remember at the moment; just woke up from a nap and the tea hasn't kicked in yet, sorry
). Test kits don't pick that stuff up, but they can build up. What effect that has, if any, probably depends a lot on the exact fish and plants you keep. Water changes are a safety policy against bad things building up into harmful concentrations.
I like keeping my tanks as natural as possible, and I used to think that water changes weren't natural. Then it occurred to me that natural water bodies have a constant influx of new water from springs, snowmelt, rain runoff. Water changes in an aquarium simulate this. Fish tend not to live in stagnant bodies of water, so water changes actually seem more natural.
All that said, I know there are people out there who swear by the no-change method. I'm fairly open minded about it. But I don't really see any reason for it.