Continuity mistake: In the 1978 flashback scene, Michael does not have 6 bullet wounds in his chest from when Dr. Loomis shot him moments earlier.
Continuity mistake: After fake Michael jumps from the 5th or 6th floor of the hospital, Tommy and the others just take a few seconds to get to the street.
Continuity mistake: The mental patient jumps from the window facing the ground. When the crowd gathers around him he's face up on the ground.
Continuity mistake: In the flashback to 1978, Lonnie has a pumpkin bag with him, but in the 1978 original, he didn't.
Answer: It was his own personal man cave, he was born and raised there. It was the only place he felt safe and secure. In just about every "Halloween" film he always returned to his home. It belonged to him. I personally believe Karen went there to get a feel for the place, trying to make sense of everything. Why does a normal kid turn into a ruthless killing machine, was it just him or was it the house itself.