It rarely happens but I have towed bicycles on occasion when a chain was
irreparably pretzeled or the derailer had snapped. Or as "power assist"
when a rider plain ran out of steam. I carry a towing rope in the left
pannier. It doubles as a tie down rope for bulky loads and to tie down
the bike in the bed of a pickup truck.
Now that many bikes have fancy carbon frames this brings up the question
whether one can still tie the towing rope to the head tube. Or is that
now too sensitive and it is better to wrap it somewhere around the stem