If you’re interested, the short version is that instances (A.K.A servers) are run by different people in different places. A reason to move instances might be:

1. My admin, the owner of the instance, has been doing things I heavily disagree with (bans, blocks, etc)

2. I don’t agree with the rules on my instance.

3. The instance is run in a country which criminalizes something that I care about, and so has to ban discussion of that thing (piracy, porn, etc).

4. I want to run a community on a specific instance for whatever reason, and so need an account there