Post-only guarantees maker treatment, which matters wherever maker-taker pricing is meaningful — crypto exchanges and equity venues alike. It also guarantees you never pay the spread by accident.
Different venues resolve a would-cross post-only order differently: some reject it, some slide it one tick back. Know which, because a silent rejection can leave you with no order at all.
Example: your exchange charges a 0.02% taker fee and pays a 0.01% maker rebate. On a $100,000 order that is $20 out versus $10 in, a $30 swing. A post-only buy at the ask is rejected rather than paying $20; posted at the bid, it earns $10 if it fills.
Related: reduce-only-order