On the off-chance that people don’t know this...

This style of bridge dates from the days when barges were towed by horses. When the towpath switched to the opposite side of the canal, the horse would, obviously, clippy-clop over a bridge and happily plod off again. Now, the automatic way to do this would be like this:

However, note that rope (black line) between the horse (brown blob) and the barge (red blob). If you cross the bridge the automatic way, it all goes a bit....

However, if you cross the bridge like THIS

it all works out fine!

Now, sure, you could trust people to remember how to cross a bridge. But there are a lot of numpties out there, and people were working extremely long days and were extremely tired. Also, the canals were BUSY. One boat getting snarled up was the equivalent of the Ever Given.

So, instead, the canal companies built Numpty-Proof Bridges.

They also had the benefit that the horse could be left to plod along on its own, rather than needing human guidance. (I have no idea how this worked. My horse would have her nose buried in the grass and wouldn’t go anywhere, if I left her to it.)