I think the idea is that you can explore the enemy base finding items/journal entries/etc without having to deal with a Zubat/Raticate grunt every 10 steps, and at the end your team will have gained some XP, you'll have found items, but you're healed up and ready for the boss fight. All while the base still feels full of enemies.
Whether it works or not is definitely going to be a question, though.
I was expecting:
You can Tera a sweeper into it's own type to stack multipliers(any given Swift Swim mon)
You can Tera a sweeper into a coverage type to hit counters(Dragons into Steel for Faries that don't expect coverage)
You can Tera a mon you need to survive to shore up a weakness(weather setters/set-up sweepers vs priority/weird defensive profiles)
You can Tera a mon to solve specific problems(hazard-setters into Ghost to spinblock for themselves)
Option #1 was already anticipated to be very good. This makes it so that you don't have to give up a secondary STAB to do it. Meanwhile, option #2 is now also really good, since you still have your original STAB and can just have insane coverage. But the other 2 options, which are the ones that involve cleverness, do not care about STAB and are therefore made a lot worse compared to the first two with this reveal.