In 2241 Michael Garibaldi was a cop on the ice mining facility on Europa. Why were they mining the ice there? Was something in it that made it more valuable?
Freshwater that has to be lifted up from a deep gravity well is expensive. Much cheaper to mine it from somewhere like Europa that is closer - in energy terms - to where it needs to go.
(Europa isn't the very best choice in that respect, but one has to assume in-universe that it has other advantages over Ganymede or Titan - such as the atmosphere, or ready availability of mineable water ice.)
Edited to add:
As Cadence pointed out, there is a major colony on Io, which according to Wikipedia "has the least amount of water molecules of any known astronomical object in the Solar System" and is therefore probably the Europa mine's primary customer.
Also, the Sol jumpgate is in Io's orbit, so any water being shipped to colonies or space stations outside of Sol (such as Babylon 5 itself!) would also need to be shipped via Io.
This makes Europa a much better choice for mining water than Ganymede, Titan, or the asteroids, never mind Earth, as it has the closest orbit to Io. Whether you're worried about the gravity well or just the distance, Europa is your best bet.
If you supply starships (and/or colonies) with freshwater, it would be a bad idea to take it from earth. The whole surface and atmosphere of earth build one giant ecosystem, and ecosystems tend to react chaotic (little causes can result in large effects - think of the often cited butterfly-effect).
It would probably have no measurable effect if you supply a single starship once with some tons of water, but if you constantly supply and re-supply a large fleet over centuries, you'd have an increasing risk of unpredictable changes to that ecosystem.
So if you are going to repeatedly take tons of water off-world permanently, it is better to take them from an uninhabitable planet or moon.
There is no in-universe explanation that I am aware of, but there might be a real-universe one.
There are volcanos beneath the under-ice oceans of Europa, and they spew out all sorts of minerals. Some of these minerals are spewed out on the surface of the moon by geysers (which have been observed).
What is not known is if there are primitive life forms (algae and such) in the oceans, living off chemosynthesis the way some lifeforms do on Earth around deep-sea volcanic vents. If this is the case in the Babylon 5 universe, such lifeforms would also be spewed out by the geysers. If so, mining the ice for nutrients would make a lot of sense. The nutrients could be used as fertilizer in hydroponic baths or could be directly processed into food.
Getting minerals and water would be a bonus, but not the main reason for mining Europan ice.