[jooz]

When using my password manager, I often provide only the password, not the user. In case of data breach they can not be exploited.

[askvictor]

Wouldn't your username generally (or at least, quite often) be your email, which would be recoverable? Might slow an attacker down a little I suppose, or prevent automated attacks.

[jooz]

Im thinking on txt files shared with leaked user and password. On those cases my credentials would not appear.

If somebody focus on hacking _me_, they already have my email, so this measure is not very effective.