When a narrowbody jet has 22 rows of 6 seats plus another row of 5 seats, that's 137 seats. If they're all full, 45 passengers are sitting in a middle seat. On one airline they got assigned that seat somewhere before entering the jetway, on SWA they get that seat either because they were willing to take a middle seat so they could either (a) sit closer to the front of the plane, (b) sit next to some particular person, (c) sit in a particular row {lounge or exit row}, or (d) because they had no other choice. But in any event, 45 people get a middle seat EITHER WAY.
Now, if the airplane has, say, 100 people on it (73% full), on SWA the first 92 of them get to DECIDE if a middle seat (near the front / in the exit row / next to whomever) is preferable to an aisle/window seat. The last 8 may be stuck with only a middle seat, unless at least 8 of the first 92 took a middle seat for whatever reason, then they'll also have some window/aisle option. AND THEY ALL GET TO CHOOSE WHOM THEY SIT NEXT TO!
With a reserved seat, there may be plenty of open seats on the plane, but when your ticket puts you next to some large or smelly or otherwise offensive traveler, and in front of a kindergardner who kicks your seat & bangs on the tray table, and directly behind an infant having a really bad day, your options to "choose a different seat" are really nonexistent. On SWA, everyone except the last few has more ability to choose whom they'll sit next to (or away from).
Having boarded SWA at the back end of the C group plenty of times, I'll say that there is a lot to be said for being able to pick your seatmate!