When you make a game, you want people to enjoy it. But how do you know if they do? Most of your friends will tell you it’s great, even if they got stuck on level 2 and decided to go do something more fun, like cleaning the oven.
Another problem is that of numbers. If you don’t have hundreds of friends, and the ones you do have are busy sanitizing their kitchen appliances, you’re not going to get enough data to base decisions on. In other words, you need a lot of people who are actually playing your game, and some way of figuring out what parts they’re enjoying, and what parts they’d rather do without.