This is a big factor of tension in the bitcoin with fiat trading because big fluctuations results into big loss ( some times to into big profit ) .
Here we can't make change our buy sell in the old rate but what if we bought at high price and after that price go down step by step .
Here we can lower the risk by only by making more buy .
Here i can explain with a example , suppose you bought bitcoin at price 3000$ , but the price get changed to 2500$ then you need to buy more bitcoin so that you can make average the price original of the bitcoin in current time .
So here you have better option to use that i told but surely you need to have lot of money in your pocket to buy more and more bitcoin with every fall in the price of the bitcoin .