I have a few of those. During the reddit exodus when the API changes went live, I had to register on a few servers to finally get a working and stable account. Issues with email or setting up captchas to prevent bots resulted in a few of my accounts not going live until a week+ later on some instances.

https://feddit.org/post/3143093

1. Multi-community simulator
2. Defederation avoidance, usually paired with accounts on special interest servers you don’t want to risk losing access to.
3. Porn and non-porn.
4. Privacy by dividing online activities between multiple accounts to make it difficult to profile you and to maintain some pseudonymity.