It’s because any change to firmware in a digital audio device will likely have some effect on the sound in some way (others with more tech knowledge than me here can explain why, but I believe that it has been explained at some length before on previous threads) - the more revealing the device, I guess the more it will be perceived by the listener?
Anyway, I first encountered such things in my early days at Naim, and I believe this was reason why Naim used to try to minimise the amount of code their CD players would use (hence the very limited features) and would code very deliberately and specifically to ensure they got best sound.
Hence whenever firmware is changed and developed Naim do tuning to ensure that performance is maintained, or even improved if possible. As for what Naim do to tune, that’s a bit of a trade secret, however, on a forum visit to the factory last year we were lucky enough to hear the effects of a new “raw” code drop and then also hear some initial tuning steps. It was fascinating really. But what really surprised me to hear was that Naim are one of the few who bother to tune after new firmware.