Do you place order before time? For example you want to buy Ripple at 15K Satoshi, Its currently at 18K and you feel its gonna go down in a a couple of days and its better to place the order if you are not online to do so at that point.
And also for selling? Do you actually buy an alt, lets say Decred at 0.01 BTC, and you want to sell at least 0.015, would you place order right away so you don't have to keep on looking how is it doing and place the order when it actually goes close to that value?
Is it wise to do that? Especially for really short term trades.
Most of the short trader do that method, Only few have patience to check hourly the price on exchange. That is very hard to do especially when you have a job in your real life.
Setting an amount for buying/selling is the best and convenient way for day traders. And it is wise decision to use that method so that you save time.