Let's say I don't want to limit myself to a single prefix, and instead use a command to change the prefix to whatever I want for each individual server I'm on.
Basically, there would first be a default prefix, for example
bl!, and if there was another bot with that prefix, I could do something like
bl!prefix .... This would then have the bot read a given text file or json, edit the prefix depending on the guild, and work with that guild and only that guild.