Someone asked me that the other day. I told em so we could pick out the work done by out of towners. Horizontal gives us a few more inches closer to the next device. Comercial has it's share of vertical. Maybe because plans spec it out are drawn elcewhere. I don't think it makes any different in the wire pulling. Many times the pipe 90's up before going to the next device. Connecting boxes with only strait pipe on wood studs creates it's own difficulties.
Anyways why dose everyone elce go vertical?
