testing: i understand it, but i don't get it (fully)

As others noted, tests can become trivial to the extent of being useless (if you get too deep, it’s like documentation. It’s always out of date).
Tests can be very useful, for things you know they can brake.

For example I have this admin page that only admins can see.
Writing a test for the simple user (a simple user must never see the admin page), will make sure I’ll never have uninvited guests.
Same thing goes for models that perform calculations, as for example a complex financial calculator or something like that.

A rule of thumb would be “If it’s a show stopper or serious defect, write a test for it”.
TDD is great in general, but in early development if you dig deep with tests, you’ll just waste time on things that will change later on. ( I know because I wasted one month on this)

(like the example tutorials mention where you test [if flash[:notice] == “Not this button, stupid”] as if you’re dead sure about the warning message).