Yeah mostly that reason. Though come to think of it I don't really run into too many situations where I need to put the day too. Normally it's month-year then whatever is inside might have the day. It's how I store stuff like photos for example, and even backups. In the case of backups there will be 7 day of week folders inside the month-year folder, typically.
Really it seems the format that makes sense all depends on the context. For something like log files or backups, it might very well make more sense to do YYYY-MM-DD. But issue is, if you're not familiar with the system or forgot how you set it up, you have no quick way of knowing what is the day and what is the month without having to analyze the existing structure. So I generally hate using numerical months because of that.