French drains can lead to a downhill slope or to dry wells or rain gardens where the extra water is held and absorbed by plants.
Depending on the volume of rain water or runoff, French drains can be widened or founded on 2 or 3 underground drain pipes. Having multiple pipes provides for redundancy, in case one pipe becomes overfilled or clogged by a rupture or defect in the piping. A pipe might become overfilled if it is on a side of the drain which receives a much larger amount of water, such as one pipe being closer to an uphill slope, or closer to a roof-line that drips near the French drain. When a pipe becomes overfilled, water can seep, sideways, into a parallel pipe, as a form of load-balancing, so that neither pipe becomes slowed by air bubbles, as might happen in a full-pipe with no upper air space.