Just curious...

Why are people buying so much gold ?

Is it something for the future in D4 it's needed for I don't know about ?

Yes.

Because gold is the main "legal" (ingame) currency and if you want to max out your character, you need to reroll your gear, which starts to cost millions at a point.

Also the top items cost millions, check out here: https://diablo.trade/listings/items