From the Neopian Times Issue 297:

Q: TNT, I was just wondering... why did you implement the "accept or deny" sent items thing? To me, it seemed fine like it was, and now... well, to me, it just sorta seems like more of a strain on servers for no real reason. Glad to hear from ya!

TNT: There were several reasons for it, actually.

First, scammers and such like to send cheated items to legitimate users in a (generally ridiculous) attempt to frame them. Now, a user can simply deny any suspicious items and have no fear of accidentally profiting off such wares.

Also, it stops users from maliciously spamming other accounts with items like dung to clog up their inventory.

There are also users who like to send unwanted or suspicious items, then activate the Neofriend only barrier or self-freeze to prevent the item from being sent back. (Why, we're not quite sure, but they do it.)

All in all, it gives everyone more control over the items they receive, which is better for everyone.