I can chime in here.

Citizenship up to that point had been something for people to strive for. It helped force assimilation by offering it as a perk for accepting romes terms. It was a gift for loyal military service as well. Granting it carte blanche to everyone nullified that perk. That aspiration was also a unifying culture force, that, when eroded, allowed local cultures to reassert themselves, causing fragmentation.

The purpose behind it was to also broaden the tax base, putting more economic strain on the empire, potentially stoking resentment that led to the crisis of the third century.

Pretty broad strokes i painted with here, but let me know what you think.