i always put my PC on something, usually my table. I've got 2 reasons for that:
1. when i was a kid, we had a pipe burst at night, and i woke up to see my flip flops floating about 10cm above the floor. so since then i try not put put things on the floor. only things on the floor are my shoes, and my dog's beds.
2. easy access. since i lived in a time, where disks were needed to be changed a lot (first the big floppy ones, then the small ones, then CDs, then DVDs), and then I needed access to the USB ports (college homework is so infuriating), i wasn't in the mind of bending down. i was lazy.
3. ok, so 3 reasons: i used to tinker with my PC a lot. it means that i opened the case many times, and put cards in and out every now and then. having it laying on the round doesn't help at all
4. i meant 4. 4 reasons: airflow. i'm used to put the PC where nothing obstructs the airflow, and it can cool itself better.