It is opinionated question of why there is such percieved increase of automated testing. Especially automated testing done by developers, not testers. Automated testing is as old as software development itself. It might have something to do with raise of agile or maybe with percieved success of automated testing at big companies like Google and Netflix. Managers and businesses might starting to realize it is actually cheaper to invest 25% developer's time to create good automated tests than 100% of testers time to test the application every time realease is needed.
One question you pose is what is purpose of tester in this environment. First, even if testers create automated tests, the tests are often inneficient. Testers often only interract with the application through same API as user does, which is usually some kind of GUI. And tests going through GUI are notoriously hard to implement, slow to run, and easy to break. Testers don't have access to APIs that developers have, nor will they make use of fast feedback the tests provide.
At the same time, testers "clicking" step-by-step through GUI to test according to some kind of test protocol is almost immoral. I once heard it compared to modern day torture. Being mind-dumbing and monotonous work, no sane person would wish on someone else.
Instead tester's primary job should be to actually break the application with novel ideas. Tester should not be testing if application is working as it was working week ago. That can easily be automated. He should be testing if there are ways the application can be used that produces incorrect or unexpected results. Tester should be emulating a user who hasn't read the manual and doesn't know how to use the application, but still tries his darnest to make it work, somehow.