Splitting up the crystal malt adds complexity, but it's a flavor thing. Just like with cooking- you may add chili powder and onions to your chili, for more flavor. Well, brewing is the same way.
Lighter crystal malts simply taste different than the darker ones. Lighter ones are more caramel sweet, while the darker ones taste raisiny. So, mixing it up is good, kind of like adding both salt and pepper to a dish. Salt and pepper and not the same thing, but both are spices anyway. Caramel/crystal malt in different colors is the same way- both crystal malts (spices) but not the same.
Carapils is a crystal malt, and so in recipes already with a good proportion of crystal malt, it's not necessary.
actually, it's not "necessary" in any beers. I don't even have any on hand. The only time I use it is in some very light beers, like a pilsner, where I want to keep the beer mostly base malt and then I'll use a bit of carapils (since it doesn't add color or flavor) for head retention.