You put your property on the market and an acceptable but not-perfect offer comes in. On the "a bird in the hand is worth two in the bush" principle you want to accept the offer even though it's not ideal.
Perhaps it's not perfect because it's subject to a suspensive condition - common ones give the buyer time to sell his/her current house or to obtain a bond. In both scenarios your sale will fall through if the buyer is unsuccessful within the stated time, and if that happens you are back to square one after a long and fruitless delay. Bear in mind that that delay could be a protracted one depending on what your sale agreement actually provides - normally no less than 30 days to get a bond, sometimes several months to sell an existing house. That's a lot of very valuable marketing time lost - and you'll never know for sure whether you just missed out on that "perfect offer".