Guests may try to book for someone else (a family member, friend), and most mean well. For example, adult children might book on behalf of a parent visiting from out of town, and it's better for everyone to have mom stay at an Airbnb than on the couch (am I right!?)
If you're uncertain about whether or not you want to allow this, feel free to reach out to the guest and ask for more information. Booking on behalf of another person can make trip logistics difficult for you and your guest.
If you are comfortable with the situation, woohoo! If the booking is on Airbnb, ask the booked guest to relay all check-in instructions and house rules along because you strictly use the Airbnb app to communicate, and their relative/friend won't have direct contact with you for information.
If you are uncomfortable, you can reach out and let them know you are unable to accept a booking for someone else and politely request that they cancel.
On Airbnb, this is against their Terms of Service. If your guest doesn't cancel on their side, reach out to Airbnb support and they will cancel the reservation for you - make sure to confirm it is penalty-free.