Its the fact that a signature is required at the point of delivery, and therefore it is a trackable item - comes in handy should the buyer dispute delivery IE:

Buyer: I havn't received my shit blah blah blah

Seller: Whats the tracking number I gave you

Buyer: JJ201005584x is what you emailed me

Seller: According to to that number you signed for it and here is your signature on the royal mail website.

Buyer: Oh cock, I did not try to rip you off at all and claim money back fraudulently


Or something along those lines anyway