The shell colour is the defining characteristic of the breed; however, Maranses are also an excellent dual-purpose breed that has its own advantages over other dual-purpose (eg Australorp, Wyandotte, Sussex, Plymouth Rock):
* Light-bodied and economical eaters
* surprisingly delicate on gardens
* relatively early maturing in both laying and cockerel readiness for eating (a 16 week Marans is good eating. A 16 week Wyandotte isn't even worth bothering with).
* a good-sized egg for the size of the bird
* very regular layers
* only mildly broody but good mothers if they do brood
This is in comparison to the relatively heavy-bodied dual-purpose breeds who tend to mature later (cf Wyandotte, Plymouth Rock, Sussex), heavy eaters (Sussex), can be irregular layers (some Sussex, Wyandotte due to broodiness), or can have surprisingly small eggs for their size (Australorp).
They're best compared to Barnevelders, which which they share a lot of characteristics - and are, again, known for egg colour among their characteristics.
And there's Araucana, of course - blue eggshell, but otherwise - just another chook.
You might as well say "That's all Wyandottes are known for? Pretty feathers and a round shape?". It really comes down to - each to their own.