To answer your first question, train as you like. (Every Friday is the training update, which lets you know whether or not your players "popped" in a skill. However, to receive enough training, a player needs 48 or more minutes at a given position to train a given skill.)
For example, Player A plays 48 minutes at PG that week and Player B plays 50 minutes at SG that week. The training is set to Passing for PG. Player A will receive training, while Player B does not (because he did not play at PG). However, if the training was Jump Shot for Guards or One on One for Guards, then both players would get training.
There are many benefits of training players.
Benefits of training:
- Player can fill a specific role in your team (such as a scorer, passer, etc.)
- Save more money than if you just buying everyone
- More training = more skills = more money. Who doesn't want more money?
- Learn more aspects of the game
There are other benefits, but I think it would be better to list these simple ones for now. Good luck to you, and welcome to Buzzerbeater!