This question already has an answer here:
I have very, very rarely gone to a site solely to reset my password. Why then, after going through a password reset process, does the conclusion of the process ask me to login again with the new password?
What is gained by this, instead of just passing me along to the "you have successfully logged in" portion of the site, once I have successfully changed the password?
I have seen this pattern used in quite a few well known sites, so I assume there must be some good reason for it.