First off, apologies if this is answered somewhere in a book. Me and my group mostly use d20pfsrd for convenience/financial reasons so it's entirely possible that I've just missed something.
On d20pfsrd in the Gamemastering page they recommend doubling the amount of wealth given to the players if they are playing at a high-fantasy point buy (20 points). They don't really give an explanation as to why though. Why would they make a suggestion like this? Does anyone here run using that rule?
It's a suggestion from the core rulebook, iirc. To be honest, there's no particular reason to tie point buy to wealth level, and there's nothing wrong with running high point buy with normal wealth or normal point buy with high wealth. I would strongly discourage running low wealth with low point buy, as this vastly disadvantages non-caster classes.
It depends largely on the look and feel you want to have for your campaign, but it's not set in stone. High-fantasy usually means the characters are at an epic level compared to their surroundings, or are equally epic to their environment. More wealth means they can purchase bigger and better things. Again, you don't have to follow the gamemastering guide exactly, I'm personally running a 25 pt buy campaign using normal wealth guidelines.
I'm guessing it's simply because behind the scenes math says that a character with a 20 PB (spent appropriately) with x amount of wealth (spent appropriately) will hit or get hit by CR appropriate monsters within acceptable thresholds (also saves, damage, and other numbers that matter.)
So if you raise or lower the PB you might leave those thresholds, but you can always notice "characters are struggling or are doing too well" and scale the loot appropriately to get mostly the same effect. This is especially worth ignoring if you choose thematically appropriate rather than CR appropriate monsters. There's no reason to assume players will spend their points/gold in the most sensible manner, either; but when you're designing the game you have to make certain assumptions.
I'm guessing it's because they think the people who like playing at high point buy are the same people who like having tons of magic loot.
Not a very good idea, really, as the CR system gets less useful the further players get from the default gameplay assumptions.