If you want to start trading only with bitcoin and fiat then it will not be constant all the time as sometimes the price come to stability and sometimes a drop down occurs in the price so you have to trade with a number of coins and it is seen normally when one coin start to decrease then any other at that time increase in price so as a trader you will start getting profit from that second coin and with that your profit will be fixed for you.
Sure, profits in trading cant be constant because of the coin's volatility. Just like every other trade, fixing the income will help business to avoid extravagance and excesses can be conserved or reinvest.