Why did Holmes put
his ear to the ground​

One of Sherlock Holmes's defects—if, indeed, one may call it a defect—was that he was exceedingly loath to communicate his full plans to any other person until the instant of their fulfilment. Partly it came no doubt from his own masterful nature, which loved to dominate and surprise those who were around him. Partly also from his professional caution, which urged him never to take any chances. The result, however, was very trying for those who were acting as his agents and assistants. I had often suffered under it, but never more so than during that long drive in the darkness. The great ordeal was in front of us; at last we were about to make our final effort, and yet Holmes had said nothing, and I could only surmise what his course of action would be. My nerves thrilled with anticipation when at last the cold wind upon our faces and the dark, void spaces on either side of the narrow road told me that we were back upon the moor once again. Every stride of the horses and every turn of the wheels was taking us nearer to our supreme adventure.

Our conversation was hampered by the presence of the driver of the hired wagonette, so that we were forced to talk of trivial matters when our nerves were tense with emotion and anticipation. It was a relief to me, after that unnatural restraint, when we at last passed Frankland's house and knew that we were drawing near to the Hall and to the scene of action. We did not drive up to the door but got down near the gate of the avenue. The wagonette was paid off and ordered to return to Coombe Tracey forthwith, while we started to walk to Merripit House.

Explanation:

The reason why Holmes put his ear to the ground was beacuse his boss told him to. Also so he kept his ear to the ground, so that he would be the first to get a new idea.