The squirrel burrows in the trunk and sucks the slurry of the tree. Because the pulp of trees is mainly water plus some sugar, the squirrel's goal is water or sugar. And because there is no shortage of water sources around the trees, squirrels don't have to spend so much burrowing to get water. 
Negative general conclusions through counterexamples.
 the purpose of squirrel burrowing is to ingest sugar.