In some games characters can have multiple classes (multi-classing) that are used simultaneously, relieving some of the restrictions of systems that allow only single class, but still ultimately suffer from the same limitations.

Actual limitations between classes may be minimal, but still significant enough that playing outside the chosen class is extremely inefficient.

This limitation can be less artificial by forcing players to join guilds or similar organisations that are the only source of learning anything useful beyond some basic set of skills everyone has.

Most obvious cases of class-based systems are that you can't learn anything without having a class and/or there are items only usable by certain classes.

Useful in some types of multiplayer games where people can have clearly defined roles which others can easily identify.

In eastern/console-style RPGs each unique character represents a class rather than the class being something they've chosen or can choose. Often accompanied with the player having no choice of class for the protagonist.