When it was there the river behind it had a higher water level. I'd say about 2 feet above the canal level. The castle moat which takes it water from further upstream had a much better water flow through it than it does now. I think that was why it was there.
The council, EA and BW couldn't agree who was responsible for it so it got removed. I think it needs to go back as it would
A - stop any future attempts by narrowboaters to go under the bridge (whether they'd fit or not!)
B - make a better water flow through the castle moat which it does need..