Classes in Project Eternity are meant to provide a general framework for character types. Different classes excel in different areas, but the framework can be extended and elaborated on in a multitude of ways to create characters with unique capabilities. If you see a fighter, chances are good that he or she is going to be able to take a lot of damage, but that's about all you can be sure of. If you see a wizard, he or she probably has some hard-hitting spells that can cover a large area, but his potential list of capabilities is vast.
If you want to create a wizard who wears plate armor and hacks away with a broadsword from behind a heavily-enhanced arcane veil, we want to let you do that. If your idea of the perfect fighter is one who wears light armor and uses a variety of dazzling rapier attacks in rapid succession, we want to help you make that character. So it's good to think of Project Eternity's classes as being purpose-ready but not purpose-limited.