The user registration record can be easily extended by 3rd party addons, and often contains useful information such as suggesting the users signed up with a VPN or proxy service. We don't want to have to review 10-30 successful registrations a day, so will often only notice after they run into issues with the forum rules which can be weeks to months after the initial registration.
At the same time, we don't really care about rejected user registrations nearly as much as being able to check successful registrations after the fact.