I don't pull an expander ball through necks, but instead expand in a separate operation with a Sinclair mandrel system.
SINCLAIR GENERATION II EXPANDER DIES | Sinclair Intl
SINCLAIR EXPANDER MANDREL - OVERSIZED | Sinclair Intl
Expansion before bullet seating is important and can reduce loaded runout over bad or no expansion.
It pushes neck thickness variance outward for straighter bullet seating.
It can establish very consistent neck tension, combined with the proper bushing size for necks.
It reduces bullet seating forces, which allows more consistent seating base to ogive.
Bullets should not be used as expanders(they aren't hard enough).