I can imagine they might in the case of connections - eg where the connection is 45mins and a later flight gives them 2 hours. They might well book the 2 hours hoping still to make the earlier connection.

If your flight is not flexible then often airlines won't allow you to board an earlier flight - even if you think it might be to their advantage to let you do so. I remember joining a queue in Lisbon for TAP air enquires. Almost everyone in the queue was asking to transfer to an earlier flight. The answer was always the same "sorry Sir your ticket doesn't allow changes unless you pay this fee etc".