It isn't necessary, but it's a welcome feature, just like in fighting games, you don't absolutely need the training mode, you can learn playing matches, but it's a lot more easy to test your options and practice combos and links on training mode.
And it's not about executing moves, but combos and skills with different abilities and a strict timing.
If you take StarRave for example, there is nothing complicated about pressing O, X and O, just tapping the desired style on the d-pad, it's a fairly simple execution, now tell me anyone who can do it on their first try consistently, it's very hard because of things other than the complexity of the moves themselves. And in that case, have an opponent with infinite life helps a lot, you don't want your enemies dying all the time, before you can master it, and you also don't want to make a lot of progress in the game just trying to learn one technique, instead of trying to play stylish, you want that SSS rank, you want to look good killing your enemies, not just attempting one skill only and failing most of the time, and then all your enemies are dead.
I really can't see a reason for anyone to think of training mode as a bad thing. People loved it in Bayonetta, people loved the modded training mode in DMC4 for PC, why is this one a bad thing?