Chasing much higher profits, that's what usually the purpose, yeah we are paid thru bitcoins, however due to btc price drops for a couple of weeks it has been observed that you can have higher returns to mine directly on alts then dump to btc. this is very likely true to people that can't wait for price pumps due to operating expenses.
If that was true more people would be bying at nicehash causing the order price to rise
increasing the return for miners/sellers. It's a free market.
Nicehash has the advantage of simplicity and predictability (no luck, orphan, exchange volatility issues)
which could cause a small affect on relative profitability.