Why are there Classes?

[xechnao]

4e is a more class-based system than 3e because multi-classing has been made much more limited. You can no longer build anything resembling a fighter 1/cleric 1/rogue 1/wizard 1/ranger 1 as you could in 3e. This is a good thing imo. If class abilities can be mixed and matched too freely then you no longer have a class system.

In 4e "classes" (or "builds" as I think is a more appropriate wording) are not so different among themselves, in respect to 3e, so your point is moot.

 

[Glyfair]

It is to encourage a cooperative game and class roles.

I agree with this. When people ask "what makes a game D&D" I think that a niche drive cooperative party is the central conceit of D&D.

3E probably went the furtherest afield from this concept. I think 4E was designed to bring those roles back.