It doesn't make your password for any particular site any harder to guess. It does avoid reuse, without needing to sync or otherwise remember N distinct passwords, which is worthwhile.
Reuse is the main reason why theft of password dbs (as distinct from just compromise of the site) is a problem for the user.
Reuse is the main reason why theft of password dbs (as distinct from just compromise of the site) is a problem for the user.