By splitting from the bitcoin blockchain, the authors of said coin have no easy way to 'premine' or get any other kind of 'first in' benefit without it being visible in the transaction history (like this is).
That should help the coin succeed on technical merits alone, rather than PR/marketing stunts and pump and dump schemes which work much better on new coins.
It also prevents 'coin inflation', where value gets split across a potentially infinite number of coins. By splitting from the original blockchain, businesses which don't want to take a risk on which coin/coins will succeed can simply demand equal amounts of every coin (or take payment in one coin, and then diversify into equal amounts of every coin). People who hold coins take no risk if additional coins fork off.