runhaskell can run whatever Haskell interpreter you’re using on your system—it does not necessarily have to be GHCi. For example, runhaskell could instead point to runhugs or runjhc or whatever, if appropriate.
Essentially, it’s just a nice alias. By using it, you make it much easier to change which compiler you’re using in the future. After all, having runghc run jhc instead would be confusing!