That s when keeping on the floor can be a bad idea.
Computer on floor bad.
You ll collect more dust on the floor requiring cleaning more frequently.
Not necessarily bad but not the best either.
Of course if you have short carpet on your floor the pc may be able to sit on that carpet with plenty of airflow underneath.
On the floor keeping your pc on the floor is widely considered the worst place due to the amount of dust and pet hair that can get inside your tower especially on any type of solid flooring.
My pergo covered kitchen floor which has withstood every form of punishment two children can exact showed a small divot where the laptop landed.
For example cigarette smoke and pet fur will exacerbate those problems and putting your desktop pc on the floor will ensure more of that dust hair and debris will get sucked into the intake.
An ex girlfriend of mine kept her computer right on the floor and when we moved from it hardwood floors to carpet her computer literally would not start while sitting on the carpet.
I slid a piece of cardboard under there and it was fine.
It s bad even if air doesn t flow into or out of the case here however if your pc sinks into the carpet a bit heat can build up underneath due to the lack of airflow.