Fleetguard prime customer is Industrial Diesel Generator engines just like CAT LLC is being marketed by Allied.... same is with the Lube Oil.... Cat is packaging CAT oil with Mobil as a vendor on the back and Cummins with Valvoline....
I understand what you mean.... "why pay for the water" but usually as you know here due to the wrong practice to mix the concentrated coolant with tap water full of high TDS i.e.Ca carbonates and sulphates / minerals in it.... so a hastle free pre mix solution with demineralized ( not diatilled) water is a better option for a non technical lay man type owner.
The ones having RO plants do not need / use premix coolants in their engines.... They use the concentrated chemicals mixed with RO water.
For our automobile diesel engines which are small, one can use any company's ethylene glycol coolant in small packing but due to suspecious 2 nob coolants available in the market as mentioned by some one above it is shared as an option along with CAT Coolant.