Its to allow them to continue monitoring his activities going forward, without the need for annoying warrants and physical searches.

I think the point is that they could have just installed the malware since they had physical possession for an entire year.

And the counterpoint is that he'd obviously got a new computer and any further surveillance would benefit from access to that one.

That's a good point. I was assuming the old hard drive was loaded on old equipment but that's not necessarily the case.