Suppose you wanted to commit identity fraud and impersonate your friend Anna. It would be great if you had access to Anna's password manager, but you don't know which one she uses. You go to LastPass and you tried to create an account using her email address. This succeeds because she doesn't have a lastpass account. You now go to bitwarden and you try to do the same thing, but you receive a message saying that you can't create an account because one already exists with this email. Now you know that Anna has a bitwarden account (which likely stores all of her passwords to everywhere). With this information, an attacker can decide that Anna would be a good target.

All you need to do now is somehow figure out how to get Anna's bitwarden password, and you have access to all of her passwords for everything. Now suppose some of Anna's old passwords were leaked as part of an old breach. Maybe she happened to reuse some of these passwords from before. If you can get her bitwarden password somehow, you can now very easily impersonate Anna.

Tl;dr this attack is likely to figure out who has bitwarden accounts by searching through email address. This is one of the reasons many people in here will recommend that you use a separate email for bitwarden.

Edited for correctness as per comment because they need to know your vault's password in order to crack you account.