First of all, let's examine why a game would use more than one currency in the first place.
Having more than one currency makes it far easier to balance. Each currency forms a closed ecosystem and allows you to experiment with it without too much influence from other game systems which rely on other currencies. For example, when you accidentally create some imbalance in your game which floods the game with currency A, any features which rely on currency B are unaffected. Similar, when you add a feature which greatly increases the value of currency B, that will not affect the perceived cost of anything which is bought with currency A.
Now how does this play out when you have a currency in the game which you sell for real-world money?
When you have a cash-currency, you need to be very careful not to devalue it. When you create an alternative way to obtain the cash-currency than buying it with real-world money, that will affect your revenue. When you screw up and your players find a way to acquire it far quicker than you intended, the result will be a notable downward spike in your profit chart which lasts until you find a way to fix it. That's why most games with a cash-currency usually offer either no way to obtain the cash-currency ingame, or only a very slow, tedious and very inefficient way to make the player notice that the cash-currency exist but not give them enough to acquire a notable amount of it with any reasonable effort.
However, many players perceive it as unfair when certain important items can only be acquired by investing real-world money. The description "pay to win" is often used in that context. A common marketing slogans used by games which try not to be "pay to win" is "Everything non-cosmetic can be obtained through regular play". The only way to do this without devaluating the cash-currency is by offering an alternative currency to buy cash-shop items which is acquired through regular play.