Key currency is a concept first used by Professor Triffin from Yale University in 1960s

At that time key currency were America's dollar and Britain's pound

Under the gold  standard, certain currencies that are internationally accepted, may be used as currency to compensate for the shortage of gold. The U.K. was the only country to represent, but after World War II US became the representative key currency.

Currently, currency that is treated as a key currency is the dollar, the euro, and the yuan.

The main reason why dollar and euro became key currency is because they can be used internationally thanks to their global status. Almost every country accept key currency with the thought that they could use it elsewhere.
However, the fundamental reason why people came up with the idea of key currency is because of the fact that various kinds of countries use different kind of currencies. Suppose that their is only one nation in the earth, who would think about different kind of currency?

The game ecology was just like that, one game world was only consist of one single game, there was no interaction with other game world. This sounds  not so bad because game always existed as sole world. Therefore binding up different games into Bryllite platform will be new trial and in that game world BRC will work as key currency.