There is also a technique in modern submarining that would not be difficult to do with WWII technology, called "layer anchoring."
There is a place in most seas called "the layer." It's the depth at which the surface thermal conditions of the ocean give way to the thermal mass of the ocean, causing a slight difference in density. For depth control, one must match the boat's overall density (buoyancy) with the density of an area just above the layer, and when everything settles out, the boat will try to find this zone. This is probably what the LI took advantage of.
As to why one would do this, it's relatively simple:
It makes the boat as absolutely quiet as one is willing to be (ventilation fans secured, order the crew to the bunks).
It makes the boat very, very energy efficient.
And, if the layer is so kind, you may be in a spot that refracts active sonar around you
For more info on the layer, just google "layer depth"