You are giving Bob the ability to unlock it himself. You temporarily add him to "the group that can enter my house" and then, without you being involved, Bob can get in or, more importantly, lock the door if he leaves when you aren't there.

My dad has a casita on his house that he frequently lets people use. Dealing with the key is always annoying, though. With this, he could say "My son will be here on the 10th-14th, my nephew will be here the 21st and 22nd, etc" and never have to worry that the key gets lost or copied.

Now they just need to tie in with alarm systems so it will deactivate the alarm at the same time it unlocks it (or at least provide a temporary code that the guest can use).