The only time I found an advantage to booking round-trip was to Punta Cana. For some reason, there was an extra $20 (or so) tax or charge included in a one-way return (which may indicate someone moving from the country or staying there long-term) vs. just going on a vacation.
Other than that, more relatable to the OP's question, I've often booked just the outbound or inbound flight, and waited before booking the other flight.