When you think about how much effort goes into designing, developing, and then testing a video game, the amount of time it takes to produce a polished product is absolutely insane.
Just look at an open-world behemoth like The Witcher 3: some poor soul has had to trek through every inch of that vast map looking for bugs, and before that, another person had to think about where to place each building, NPC, and hidden treasure. That's not to mention the thought that goes into crafting solid mechanics and an engaging story, which most games would fall apart without.
With so many moving parts to take care of, there's always the possibility that the developer will put something in their game that doesn't always result in the player having a good time, whether that's an annoying or frustrating mechanic, a ridiculously tough enemy type, or a design flaw that halts (or even erases) progress.
And things like this can be easy to stumble across - often accidentally. By simply playing through a game normally, you might inadvertently run into a situation that destroys your enjoyment of the experience. How fun.