One possible explanation is that, simply put, mutilation is a lot less permanent than death. If the emperor has someone executed, even for something like treason, this can pretty quickly degenerate into a kind of vendetta situation. Family members of the deceased rising up to take revenge for their family member causes a lot more problems than even just one rebellious noble. Mutilation, on the other hand, while it's still a horrible fate, at least (or at least theoretically) leaves the person alive, and creates a useful message. I very distinctly remember reading once that the Byzantines justified this kind of punishment because it was more "humane" than execution, but I am...not entirely convinced by this argument, at least not until I can remember where I found it. Whenever Psellos, for example, talks about political mutilation it's pretty clear that he does not think it's in any way a preferable outcome, and whenever someone gets blinded he very often adds on that they died soon afterwards.

Most political mutilation in Byzantium - blinding, amputation of hands, nose, etc. - was meant to leave obvious results. This would act as an outward indication of past transgressions. Blinding, in particular, could be executed in a number of ways, and apparently even to varying degrees: the Oxford Dictionary of Byzantium cites at least one general who had been blinded but then subsequently managed to lead an army, though the entry very unhelpfully does not give his name. Political castration occurs in a number of circumstances, so the "tells" resulting from pre-pubertal testosterone deprivation weren't necessarily present in everyone this happened to: Patriarch Germanos I was castrated as an adult and is thus usually depicted with a beard (though not always [on the right]), whereas Ignatios I, another eunuch patriarch, was castrated at a much younger age, after his father Michael I was deposed, and is thus usually shown beardless (though, again, not always).

This is a very complicated subject, and I don't know of any studies specifically on mutilation in Byzantium, but they may well exist and I'm just too squeamish to have really gone looking for them! I (pretty obviously) know about this mostly through its connection to eunuchs, so my actual understanding of this practice is somewhat limited.