At a more general level, the exchange rates expressed by any currency pair that does not involve the US dollar are called cross rates. Thus, this broader definition implies that the exchange rate of the currency pair GBP/JPY would be a cross rate, regardless of the country in which this quotation is being made.
A cross rate is often used as a tool in currency trading by investors. The comparison of the current value of one foreign currency to the value of another foreign currency is considered as an extremely important indicator for currency trades. This indicator provides investors a helpful method of tracking the impact of various events on the value of the currencies that are being traded.
This data is thus used for predictions of the future performance of currencies in the open market. Of course, it is assumed in such cases that the said events continue to impact the performance of the currencies that are under consideration.
Cross rate trading has become popular among investors since it provides them an opportunity to hedge against currency risk. Speculators also find cross rates useful since they can make profits from interest rate plays and exchange rate movements.
Since the value of a currency changes very rapidly, cross rate trading needs constant monitoring. Also, cross rate trading is nearer to speculation and is, hence, extremely risky.