The problem with free to play (f2p) games is that they have to make money from something else since you don't pay to play (p2p). Of course that means they monetize the games with in app purchases (IAP). But how games use IAP varies widely. The best of them limit IAP to non essential elements of the game, like purely cosmetic items including different outfits or skins, extra mounts/pets, etc. You see that a lot on PC games. PC gamers are less tolerant of IAP abuses because of the history of PC games and the number of good games you can still get on PC that don't go crazy with it. But the trend has been increasingly towards developers getting greedy and doing two things that are more abusive to go for the cash grab.
The first is the pervasive use of loot boxes. Most loot boxes are essentially a form of gambling since you pay for the loot boxes but do not know what you will get in the box; the rewards are random. Most of the items you can get are pretty worthless (and in some games outright crap) while only a few items are really what players want. So if you want a particular item, you may have to buy a lot of boxes to get it. And the animation and sounds of opening the loot crates strongly mirror using slot machines. They are designed to play off the same emotional response people have using slot machines. It's not surprising then that some people have developed gambling addiction like symptoms when using them, causing them to spend absurd amounts of money on the crates. The problem is so bad in some games that several countries, including Belgium, the U.K., the U.S., and China have passed or are considering legislation that would regulate or ban the use of loot boxes at least to people under a certain age, usually age 18.
The second trend has been going for pay to win (p2w) grabs. P2W games are designed in such a way that the game is very annoying to play (or even realistically impossible to play) and progress past a certain point unless you buy items to get around it. So making a game very grindy but offering you ways to avoid the grind by buying something with real world money is a one very common way to do p2w. You see p2w far more often in mobile games. Indeed the reputation of mobile games generally is that most are pretty bad and are filled with the worst abuses of p2w. While a few mobile games are good, most are pretty much trash. But they are cheap to make and can make a lot of money as long as they can hook a few people who will pay a lot for IAP, known in the game industry as "whales". As long as they have enough whales the worst game developers won't care that the rest of the players think the game is terrible. It's a sad state of affairs and its why I avoid mobile games altogether. A lot of gamers, especially older ones that remember gaming before f2p games absolutely hate p2w. But there are enough gamers, particularly teens who have only ever seen a game market that has these games, are willing to pay up for p2w stuff. Its even worse in the Asian markets since those players have a higher tolerance for grindy games than Western players do. That's due to the history of game development in Asia.
And at the very bottom of the barrel are the developers who put the p2w stuff in loot boxes.
The game you played is one that appears to do just that: it combines both p2w elements and puts them in loot boxes. Yet it isn't the worst of these games since it came out a few years ago before the worst offenders entered the scene. When getting new games today, especially mobile games, you need to expect that most of them will have one or both of these elements to some degree. If the game starts to get real grindy and you have to buy stuff to make any realistic progress, ask yourself whether it is really worth it. You can end up spending a lot more money than you realize with IAP. They make really easy to spend the money and make it tempting when it allows to avoid hours of grinding or when you need to have the item to win and can't get it from game play at all. I wish more gamers would opt for good p2p games that lack the loot boxes and p2w elements that f2p games employ. Or even pay subscriptions for games to get rid of those elements. Those games tend to be much better, but sadly they don't actually make as much money as f2p with aggressive IAP as long as they get some good whales. And what we end up with are a lot of sub par games because players are tricked into thinking the games are "free" when in reality you have to pay out a lot if you really want to reach the top levels.