In 5th it increased assault distance out of transports and was a real, game-impacting advantage.
In 6th, When trying to avoid challenges, having a larger base at the back of a unit can make it easier to avoid being engaged at the start of combat and then slink in towards the end. While the tactic can be done with smaller bases, larger bases tend to have a harder time navigating in assaults.
With
OOP models and such, there are models which should be on 40mm for modern models which get a large advantage by being on 25mm.
There are ways to 'take advantage' of different base sizes, as long as you don't try to, it should be fine.