Hey Bullseye!
Expanding the case mouth after resizing is done on straight-wall cases for the same reasons it's done on the bottleneck cases. It makes the case mouth MORE UNIFORM in diameter to receive the bullet.
Without that case-mouth expanding step, different brands/batches/lots of cases with differing wall thicknesses would give non-uniform case neck tension. All of this would be bad on ballistic uniformity, and therefore, on accuracy.
Resizing cases intentionally makes the case mouth/neck smaller than necessary for seating a bullet. Expanding opens it back up to make bullet seating more uniform.
Back when I was young, foolish, and lazy, I played around with loading .38/.357 revolver cases without the neck sizing and expanding steps. WOW! I thought, this makes relaoding a LOT faster!
YEP, you can guess the end results: poor ignition, bloopers, bullets stuck in bore, spilled powder, and really crummy accuracy.
That neck expanding step is in there for good reasons! Use it!