You need more than just water if you're sweating
One bottle per day, in those temperatures? That's impressive: most cyclists would be badly dehydrated if they did that.
No, one 660ml bottle on my bike, refilled regularly.

On tour I never drink anything other than water, pop & beer and most of my long tours are in Spain, in summer............even in 30c-40c I don't sweat much on the bike unless struggling up a long climb, it's in the late afternoon when I stop cycling and lose the wind and my own generated breeze that the sweating starts.
In hot weather under heavy exertion an adult can need up to a litre of water an hour to maintain performance and avoid heat or dehydration injury.
I don't use anywhere near that, one 660ml bottle might last an hour, or maybe 2 or more hours, depends where I can fill up with the next potable water. I do recall a problem on the mountains before Seville, but put that down to a prodigious amount of alcohol consumed the night before as well as running out of water.......
I am not disputing some sports drinks may be beneficial but the OP was wondering if they were "necessary", I don't think that they are.