Why do countries have so much gold as forex reserve?



Well it's a combination of a few things. 

The first is that it's a leftover relic of the gold standard. Paper money used to be backed by precious metals, when this proved to be an inadequate system of currency they moved to a fiat system. The gold that's leftover from that has pretty much just sat in banks around the world. 

The second is that gold is a decent asset to actually hold. It tends to appreciate over time, is a protection against inflation, and provides some asset backing for issued currency. Despite what people think, currency isn't unpacked, it's just backed by the government representing it. The more assets said government has, the more stable their currency is. 

Third, it's kind of hard to really do anything with it. Who's going to be a buyer of 50B in gold? Transporting that can literally take decades. A lot of countries might want to get rid of it, but they're just taking the path of least resistance.

Gold - is not a bad option, yet it is a bit risky. I have some funds converted to gold, but not too much. Gold was rising till AUG 2011 and since then it is going down. I am not sure if it is a right time to invest into it.

Forex trading - is risky. Don't go into it, as it is not an investment. Trading currencies is a pure speculation. Even though I love forex and trade often, you should not consider this as an investment. It is a high risk and it takes time.

For other interesting options I could consider digital currencies, as they are very volatile. For safe investments, you can always consider a cash deposit. Some countries could give you an annual rate of about 6-7% for USD.