One is security updates and bug fixes. These need to fix the problem with the smallest change to minimize the amount of possible breakage, because the code is already vulnerable/broken in production and needs to be updated right now. These are the updates stable gets.
The other is changes and additions. They're both more likely to break things and less important to move into production the same day they become public.
You don't have to wait until testing is released as stable to run it in your test environment. You can find out about the changes the next release will have immediately, in the test environment, and thereby have plenty of time to address any issues before those changes move into production.
