When you are making a game with 100-200 or more levels and have to create each level manually, placing sprites on layouts - this is a very time consuming job. And yes, I agree, in many games this may be the only way to build levels.
However, in lots of other games (especially puzzle games, which this post is about) you can optimize or automate this task by generating levels randomly, or with data created using some external tool. And this can save you many days, weeks or even months of work.