This willow tree has been pollarded, where the branches have been pruned back to the trunk. This similar to the practice of coppicing but done above ground level and keeps the tree at a certain height.
Cutting back the higher branches invigorates future growth and encourages the new stems to grow long and straight. Trees are traditionally pollarded to provide either food for cattle (in the form of abundant leaves) or for wood (grown over a longer period of time). An advantage of pollarding over coppicing is that the new growth can be encouraged out of the reach of livestock.
Willow is maintained in this way to provide wood for use in weaving baskets, sculptures and other items. You can see good examples of this along the waterways of the Somerset Levels.
This is a traditional, sustainable method of woodland management as it can be carried on indefinitely and can even improve the health of the tree.