Gordon Brown is clearly trying to bind his most likely successor into his government by appointing him Home Secretary.
It's difficult to see what else he could have done really as I guess Miliband won't be moving and Darling has already confimed he definitely isn't moving. Johnson's appointment again underlines Brown's weakness. There was a time when he was free to appoint whoever he wanted to whatever post he wanted but that time has now passed.
He is effectively in Zugzwang.