Lets say I have set up a buy order. Now I directly want to secure the trade with a stop loss. The exchange now tells me: Your funds may be insufficient for this market order, because I indeed do not have these funds yet.
So I go to sleep and the buy order gets filled. Unfortunately price keeps falling and I wake up with a big loss.
I think for traders it is necessary to be able to set up a stop loss straight away, before an order gets filled.