Control weather as a device on the battlefield is one thing, but the thing that would really win you fame and fortune is a wider, less severe, and longer lasting one. One that let you break droughts, or stop floods.
There is, of course, the danger of its being used offensively to cause droughts or flood, but then, the D&D world would also have protective spells of some kind. Kingdom-wide perhaps, tied to the coronation, and other such things.
Widespread agricultural magic and anti-disease spells are the only thing that would lead to the low infant mortality rate of the typical RPG world. Create food and water is all very well, but there would be spells to fertilize the land, or undo irrigation-caused salination.
Jails. Courts of justice. Messages. Travel. So many things that magic would affect the instant you let it work reasonably. . . .
There's a character -- I don't know whether she's a cleric or a wizard or possibly a druid, but I do know she's going to be a hydrologist. In a world where the four elements get out hand, controlling the balance of earth and water is important.