I actually just learned this lesson recently (in the last week). I have a NAS that I use for my PCs, and it also stores my media collection for Plex, it was natively sitting on the same network as my PCs, as that’s where I was most concerned about network speed. I was having it cross VLANs for the Plex stuff, and it was only when I got a Ubiquiti switch that I noticed that traffic was hitting the router when crossing the VLANs but not when the two subnets were the same.
I’m happy that my hard knock lesson can help someone avoid that same mistake.