Refueling for what? The realities of orbital dynamics mean that Mars isn’t a convenient stopping place on the way to say, the outer solar system. I don’t have the numbers in my head but the delta-v requirements to intercept, match velocity and land negate any potential refueling benefit in my understanding. (Not to mention the added complexity vs a straight shot, this is a lot to pull off even in a sim like KSP)

You might be able to make an argument for refueling bases on Mar’s tiny moons with their negligible gravity wells for Martian missions themselves but that’s about all I can see.

What am I missing?

Unless the water is replenished somehow, sending it all off-planet would mean no one could settle Mars.