The point is that gmail or other webmail emails still get traffic and not everything is guaranteed to be filtered out as spam. There are some emails that might not be considered spam, that may well be even worse.
The idea of these temporary email address is that they have a limited life and you never go back to visit them once used you discard. With a gmail or webmail account you have to create it, register/sign up, nothing required for these disposable email addresses.