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.
It is very wise to do as most traders loses the opportunity to get into trade when it is at its lowest momentum. This feature is available on poloniex trading platform and others popular trading platform. If you want to learn how to use them then try visit poloniex and practice it or Google for others exchangers that used those features.