"National Roman Legion"

[Caius Fabius]

When the enemy presses the centuria becomes compressed. Being on the same foot to counter push is best. Keeps the men together. Men in front tend not to step apart or step out if they step together.

Stepping apart creates gappage in the ranks. A good enemy will exploit those small openings, especially an enemy who's fighting without strong command and control, with lots of "hero" combatants.

Also, if men are on the same foot, distances for thrusting remain uniform. It makes it easier to toss pila.

It's also disconcerting for an opponent to see a wall of shields advancing in step like a single minded entity.

It also helps the men stay aware of each other.

This assumes that the Romans fought in a medieval shield wall or a scrum like the Hellanistic pikemen, or SCA heavy combat fighters, instead of the looser formations recorded by historians. If each man has more than a meter squared of space in normal formation, he won't be stepping on other feet?