There is many reasons a person may choose to play solo player content which is a form of casual content. In this thread you can read some of those reasons.
Ontop of what is described in that thread, when it comes to general casual content. It’s the most important part of the game. You can only use one raid a week then it’s locked out. So that may cover a few hours a day for 1-3 days. What do you do with the other 6-4 days? Just do mythic+? Casual content is needed for everyone it works like the time off between raids. It also provides additional income to Blizzard from people who don’t like the high end content. Allowing them to make better content for those that do enjoy it.