ltc-
As @Chaley very correctly said, pads and rotor wear together, more often than not if you only replace the pads with an old rotor, it can cause accelerated wear on pads, less braking power and slight (or more( noise.
This is problematic when a pads is worn nicely to one rotor but does not provide exactly the same braking power on another rotor from another wheel.
Techinically a rotor is to be ridden until it hit 1.5mm thick, but in my experiences servicing bikes, it’s not the thickness but the wear ridges on the rotor (like a vinyl record).
I go for Magura rotor for this reason, they’re very hard wearing, less likely to have wear ridges and can go through several pads before performance get reduced (by wear ridges).
When I just replace pads, it is not uncommon for customers to return saying their braking are not great, once I’ve started quoting both pads and rotors, the number of customers returning plummet massively (same with chain; replace both chain and cassette also reduces this massively rather than just the chain, especially on 11+ speed bicycles).
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edscoble
The reason I swap rotors as well as the wheels now is because I found the brakes needed I bit of bedding in again each time you swapped. My guess is the pads and rotors don’t wear evenly and they get ridges which can fill with contaminants?