Some users use them to quickly create "alternate" accounts to bypass server bans or to stay anonymous.
The most frequent use is for malicious purposes, such as sending mass direct messages (DMs) or "raiding" servers with hundreds of bot accounts.
If you are a developer looking to test a new bot or server feature, you can:
Many publicly available "account gens" are actually malware or "token loggers" designed to steal the user's own login credentials.