Let's say your question relates to the TSA fingerprinting for flight training in the US.
If at any case in the future, you issue a second (or third, or fourth...) training request (for whatever reason you might do that. Maybe you had to cancel your original plans due to illness or so), you need to use your existing login credentials in order to be able to use your existing fingerprints.
If you sign up with a new account, you will need to obtain (and pay for) new fingerprints.
Seems pretty obvious to me, to be honest. ;-)