Not strictly true. As it applies to the main: Yes. But backstay tension affects the forestay tension on a masthead-rigged boat, allowing you to adjust the headsail's depth. (We have a backstay tensioner on our masthead-rigged boat.)
It's my understanding backstay tension has little-to-no effect on the main on a masthead rig.
If so equipped<<
I always understood the Baby stay did two things, allow the mast to bend (in the right direction) with backstay tension and strengthening the mast while running a spinnaker. Surely there is more to it than that but without this stay you're not in control of the mast column with backstay tension. All the resulting advantages to backstay adjustments are not controllable unless stays (and spreaders) are swept back, without a baby stay.