Perhaps for avoiding an attack, but has noting to do with law enforcement.
Anyone can send someone anything.
It doesn't "prove" you were expecting it.
You're not required or expected to know on advance what you're signing for.
That's absurd.
I've received many mail pieces i've had to sign for without knowing what the hell it was.
A signature merely confirms you received it.
A "controlled delivery" is used to obtain the probable cause necessary to search a location they don't have enough to get a search warrant for,
All that requires is for the package to be brought inside, by anyone. No signature required.
The latest devices have sensors that even alert LE when the box is opened, at which point they enter and search.
Requiring signature opens the door to the ability of the package being rejected (by a suspicious, paranoid criminal, for instance), so they don't require it. The package in your mailbox or at your door is far more likely to be a "controlled delivery" than your mailman knocking asking for a signature.