Gold Standard: Currency was backed by physical gold you could withdrawal in exchange for a set amount of USD.

It was removed because it has some systemic limitations that made it more complicated to recover from the great depression (it forced high interest rates to deter gold depletion. This limited access to capital for businesses--so businesses couldn't get funding to open and employ people easily at all, if you want to get specific).

In the most cynical terms, it was removed so bodies of power (the fed is not a government body) could manipulate currency availability (how many dollars are floating around) with no real check or balance. It gives them the ability to just print more money without having to have any gold (commodity) to back it.

Like most complicated things: It is both a good thing, and a bad thing it is gone depending on your life strategy.