Multipools switch between different altcoins and constantly calculate which coin is at that moment the most profitable to mine. Two key factors are involved in the algorithm that calculates profitability, the blocktime and the price on the exchanges. To make sure you don’t need many different wallets for all possible minable coins, almost all multipools now automatically exchange the mined coin to a coin that is accepted in the mainstream (for example bitcoin). Using this method, because the "most profitable" coins are being mined and then sold for the intended coin, you generally receive more coins in the intended currency than you would by mining that currency alone. This method also increases demand on the intended coin, which has the side effect of increasing or stabilizing the value of the intended coin.