What was gained by regular nickname changes?
Each gang member is given a nickname (Mr. Blue, Mr Pink etc) for each job. Only the leadership picks the names, the nicknames change with each job\event.
Members do not share real names or personally identifiable info.
The benefits include: Plausible deniability about duration of membership. Plausible deniability about which jobs\events member was party to. Compartmentalization of the enterprise.
I'm struggling to find a legal use case where this could be applied. Although political dissidents (with recent events in Turkey in mind) may have to break the law in any case.
Even if you've got good OpSec, you may reveal things about yourself accidentally over time, and with a long-lived nickname, this could potentially narrow down your identity. Things like colloquialisms/references that are culturally specific/indicative, or reference to things that indicate things about yourself by association.
This could potentially be counteracted by analysing usage patterns/language patterns to try and associate a set of nicknames as 'the same person', but it's probably hard-ish to do if communication is terse + sparse.