It was standard to have them on all the back lots in case of fires due to cheap temporary construction. Many studio lots were run very much like small towns. So if WDW was going to build a "working studio" then it would include a water tower by design. That said - water towers are common everwhere, cities, towns, counties, roofs of older tall buildings .... so leaving the tower would not have been odd, especially since the park still has two+ streets of old Hollywood architecture.
"..water towers commonly found on Hollywood studio backlots of the first half of the 1900s, originally a safety measure to douse any fires on film sets."