Because he wanted to hide or destroy something on it. Or IF he had a conspirator or encourager, perhaps they directed him to do so.
Maybe he was hiding his kiddie porn collection, who knows.
The point is, they won't know until they know and it would be procedurally stupid to let any information out at all until they are sure there was no one behind Lanza, pushing him over his own psychotic cliff.
Maybe he had porn on the computer. Hey, maybe he was surreptitiously filming his mother, and had pictures of her on his computer.
One day, she finds out, gets really mad, and smashes his computer. He retaliates by grabbing one of her guns, and killing her. Then he runs to the elementary school and kills a bunch of kids before taking his own life.
Or maybe he took his computer with him to the school, but when he drove through the bermuda road triangle, his computer was sucked out of that reality and back to his house, where it was dropped in his room, smashing the hard drive.
How do you smash just a hard drive, without first taking it out of your computer?