...added weight is for better traction. Almost all 2WD tractors will have their back tires filled with a water/radiator fluid mixture (ice doesn't work well if you want to keep the tire on the rim).
My guess would be one or more of: the above; greater heat capacity; or, something to do with surface area vs. volume meaning that at a large enough size water in a tire is far lower-pressure and easier to contain than air at a high enough pressure to keep it inflated, and/or, relatedly, something to do with heat dissipation from compressing gasses being really hard to deal with once you hit a certain surface-to-volume ratio.
[EDIT] LOL, guess all these were wrong and it's just for the extra weight.
https://en.m.wikipedia.org/wiki/Sphere_(1998_film)
that is confusingly similar to The Abyss,
https://en.m.wikipedia.org/wiki/The_Abyss
I always get the two confused.
Beet juice is interesting! lol
That isn't strictly true, but in general such equipment spends most of it time moving very slowly it it moves at all. Transports speeds do become a problem, but that is only done for a short time (or you have it on a trailer so you can go faster) so nobody worries about the issue.
IMPORTANT: Never fill any tire to more than 90% full. More solution could damage tire.