Additional reasons:

-If you have a companion pass, your companion may want to fly just the origin or just the return flight. Current system limits the companion to the EXACT same itinerary as your ticket.

-If you are traveling with an lap child, after checking in for the origin, your return flight gets tagged internally +INF, which can cause the same difficulty as a customer service phone change: future changes to flight dates/times require a phone call. This internal tag doesn't save any time, and doesn't avoid the requirement of showing proof of age in person at the airport.

-Funds from cancelled flight segments get applied differently when you use them. Funds from completely canceled tickets (like from a one way) are applied first, before partial funds (like from a price drop or a missed origin or return flight).

-Ability to cancel just your origin segment if your plans change without making a phone call. This can help the airline resell your unneeded seat. This will be even more of an advantage once Southwest implements the no show fee.

-If you establish a regular pattern of one way flights, you are less likely to draw security scrutiny even when buying last minute one ways. (note: I have not seen an SSSS boarding pass in years, but when booking last minute for others, they can sometimes not check in online, and instead get a note saying that restrictions, rules, etc, do not allow for a boarding pass number to be printed online, and you must check in at the airport).

-You are only allowed 2 active boarding passes, per PNR #, at a time. On a same-day turn with a connection on the outbound or return, you are restricted from getting your other boarding passes until you have used up one of the segments. Booking all fights as one ways avoids this hassle.