Game balance. It would be boring if everyone could do everything so the game limits what you can do by placing different abilities into each class. This means each class has its own set of strengths and weaknesses that prevent any one player from becoming a ridiculously overpowered shounen protagonist.

There's no wrong answer when it comes to choosing a class. Pick whatever you like.

[What he said wasn't completely true, but his pupil doesn't need to know that yet. Touma wants him to be able to enjoy the game without having to worry too much about the more technical stuff.]