E.g., KLM airline offers a fare lock for a fully refundable ticket (fully refundable till flight boarding):
Need some time to think? No problem: you can lock this fare now and pay later. The date and fare will then be guaranteed until Tuesday 20 December 2022 at 20:30 (local time). Save this fare for 24 hours. USD 48.00.
The ticket is fully refundable:
The fare lock option got offered after choosing the fully refundable ticket (but I assume the fare lock option is offered for non-fully-refundable tickets as well).
What's the upside(s) of paying for a fare lock on a fully refundable flight ticket, if any? Assume there's no booking fee (which is the case for KLM). Since it's a refundable fully flight ticket, I thought one could simply buy it and refund it as a way to lock the fare for free.