"I know that the train from Barcelona to Valencia is not an AVE so no need to book seats in advance. "
That's not how it works. All long distance trains (so far as I know) require reservations, not just the AVE ones. The Barcelona - Valencia EuroMed trains do, for example.
Of course, booking in advance can just mean getting to the station a bit early and buying the ticket, on the day of travel, it will come with an assigned seat reservation. You can do this with AVE too. But, the disadvantage with this approach is (a) all seats might be sold out (not certain) and (b) you'll be paying the highest price (almost certain).
"very strange"
Quite a lot about how RENFE works is inexplicable. But the trains themselves are good. I recommend you spend a few minutes reading this guide which gives a good primer
https://www.seat61.com/Spain-trains.htm
I second what David says about considering coach trip and flying for getting to and from Granada respectively.