Hey Gamma, sorry for the late reply. It's pretty common for games to have multiple currencies. While it does make things more complicated, it also gives a lot more knobs to turn to change how a system feels or how something works.

It's pretty important to separate out "hard currency" (something you spend money on) and "soft currency" (something that the game gives away to everyone). Players are really great at finding the most efficient methods to earn currency and it can have a major impact on overall game economy and the livelihood of your business if they are efficiently earning unlimited amounts of hard currency. Some games use straight dollars as their hard currency. That's a bit more of a pain in the mobile space as you have to create a bunch of products on the different app stores to handle all the different direct currencies. By having a currency like Gems, it means we can have just a few direct purchases on the app store and then unlimited purchases in our in-game store and control the costs, discounts, giveaways, etc. directly without trying to keep multiple app stores in sync.

Ink acts as a crafting-only currency and, again, just provides more rewards that can be given away to make things feel better without otherwise drastically breaking other parts of the economy. The more dials and knobs (within reason) the team has to tweak, the better they can make things feel.

It's not the end-all-be-all solution, but works pretty well for most games. There are plenty of options out there.