At Dowley Gap, the CAnal and River Trust has this to say about it:
This attractive old packhorse bridge is contemporary with the Bingley Locks and was built around 1773. At this point the towpath crosses from the north bank to the south bank.
The bridge allows the boat horse and boatman to cross over the canal, and ‘change line’ without slowing down the boat.
The bridge is there to carry the much earlier agricultural trackway Primrose Lane, which connected two different areas of farmland dissected by the canal.
The hole in the side wall of the bridge allows silt washed down the slope from Primrose Lane to be caught in the rectangular walled enclosure or silt trap.