The flat overhand bend is used to tie two ropes together when life safety is not a concern. The most common application is to attach a pull cord to a rappel rope. Because both ropes exit the knot on the same side, the knot is less likely to catch on rocks during the pull.
The flat overhand bend is less likely to catch on rocks or debris than the figure 8 bend or a double fisherman's knot.
The flat overhand bend has a tendency to capsize (roll over) under load. For this reason, the tails should always be left long.