Ginger is a unique substance. It's origin is clearly in South Asia from where it has spread far and wide. Its use in everything from the Japanese gari to ginger ale to gingerbread and ginger snaps means that it's about as ancient as spices come.
The CC would like to talk about a very specific thing. Dried ginger.
The flavors are completely different from the fresh variety and they are not substitutes for each other. You will see the dried version most often in the context of Moroccan, Tunisian, or Indian cooking.
Fresh ginger has a pungent edge. Dried ginger has an edge but it's a lot more mellow. It has a positively fragrant disposition.
The CC has seen dried ginger in two forms. One is the whole root. You cut off as much as you need and grind it as needed. (A coffee grinder rocks out just fine.)
The second is pre-ground which is fine too but you need to store it in the fridge. It has a tendency to absorb moisture and will spoil.
There's also a half-way house which is dried ginger slices that you can then grind.
One of the classic uses of dried ginger in the pan-North-African-made-its-way-to-India context is its use as a sprinkling over pulped fruit. Specifically fruit that is very sweet. Specifically mangoes.
The dried ginger acts as a tonic cutting through the absurd sweetness of the fruit. Salt is another way of doing the same and you frequently see dried ginger in this context.
Of course, its also used in innumerable spice mixtures but that's not very surprising.
Many cookie recipes from Europe also call for dried ginger not the least because it doesn't have any moisture which would change the ratios of the ingredients needed although these days the hipsters are sneaking back in fresh ginger.
However, as the CC has already stated. They are not substitutes. They are fully equals and just different things even though the derive from the same source.