They are there to allow boats to travel up and down the river. The river flows downhill, and if it’s too steep, the water flows in such a way that the water level will be too low in that fast moving section for boats to travel. The lock usually accompanies a dam or weir to raise the water level upstream, but the boat needs to use the lock to make that height transition. The only difference when a canal uses them is that it was planned out in advance, along with the rest of the canal, where as in a river we have to do some modifications.
Source: I live near a lot of locks
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