Because at its best Redis is an in-memory database. That means saving log information to it ( the exceptions are log information after all ) will take RAM. And

Because at its best Redis is an in-memory database. That means saving log information to it ( the exceptions are log information after all ) will take RAM. And at some point you will have to migrate to bigger VPS just because of that. You can manage that with the expiring keys, but some people prefer to keep their logs for long time.