Difference in cooking on top vs bottom rack?

Hello, I can't seem to find the answer I am looking for on google or this form. I watch a lot of atbbq on youtube and can't figure out why he smokes the majority of his meats on the top rack vs the bottom rack. I get that there is going to be a temp difference between the racks but is there any particular reason why I should use the top rack as the main cooking grate vs the bottom?
I also understand that you can designate which rack to cook which meat on depending on if you want it to cook faster on the main bottom grate.
I guess a simple way of asking is, why do I see most people smoking meat on the top shelf when you have the bottom "main" grate for cooking.
I also understand that you can designate which rack to cook which meat on depending on if you want it to cook faster on the main bottom grate.
I guess a simple way of asking is, why do I see most people smoking meat on the top shelf when you have the bottom "main" grate for cooking.