Imagine, when Brian does not get any response from Brianna, he decides to buy another condo. He doesn't want to do so, however, if his offer to Brianna is still open. He doesn't want to end up contractually obligated to buy two condos when he wants only one.
Brian can make sure that her offer to Brianna terminates by revoking it.
A revocation of an offer is a manifestation by the offeror of an intention not to enter the bargain the offer proposes. Revocation is effective when the offeree receives the manifestation. See Restatement (Second) Contracts ยง 42.