Did some gorgeous model just send you a friend request?
You search your memory but just can't seem to remember the person trying to add you as their friend.
If your spouse or significant other is trying to test your fidelity in an unscrupulous manner, they may resort to creating a false profile using an attractive profile picture to entice you into becoming their friend so that they can further test you by trying to get you to respond to their suggestive posts or chats.
They could record this information with the intent of using it against you later.
They may send out random friend requests to huge numbers of people before they find a willing victim.