Why did Sherlock Holmes bang the stick on the ground outside Wilson’s house?

At the end of the story, Sherlock Holmes explains why he banged his stick on the ground in front of Wilson's home. As he states:

I was ascertaining whether the cellar stretched out in front or behind.

He determines the cellar is not in front of the house.
When Holmes rings Wilson's doorbell, the pawnshop's assistant opens the door. After making inquiries, Holmes learns this man is a notorious, cool, and daring criminal. He and his cronies are digging a tunnel to the nearby City and Suburban bank behind the shop in order to steal the bank's gold bullion and take it out through the pawnshop. The worn, stained look of the assistant's knees confirm's that he has been "burrowing."
The Red-headed League was simply a ruse the criminals devised to get Wilson out of the shop so that they could dig their tunnel and carry out their robbery without his knowledge.