Lets look at this way....
Example: 3x3x6 tent and 400w HPS.
Wit above setup and growing hirizontally/flat with a hood,you only have a 3x3 or 9 square foot canopy.Using a hood (even aircooled ones),unwanted radiant heat is blasted back down to your plants.Light from the top side of the bulb is reflected of the top of the hood and passes back through the bulb,killing lumen.efficiency,and if it's an aircooled hood then this light also has to travel through the glass at the bottom if the hood which is known to cut lumen efficiency of the bulb by 10% to 15% (that's a double whammy).
Now,by taking that 400w bulb and hanging it bare bulb down the center of the 3x3x6 tent we are cutting out any light and radiant heat being reflected of a hood and having to pass back through the bulb or any glass (which kills lumen efficiency),and plants can be placed closer to the bulbs without fear of overheating or light bleaching.
Also,if you use one plant per each wall minimum (four plants total),each wall is it's own 3x3 or 9 square foot canopy,so you are quadrupling your effective canopy in.the same space.
Just place a small desk fan on low,centered on the floor pointed up at the bulb with your carbon filter/exhaust fan combo above the bulb.This creates a small wind tunnel if you will,carrying any heat thrown off the sides of the bulbs allowing you to get your plants closer to the bulb and really blast them with all those lumens.
Here's a goid thread to read.
https://www.rollitup.org/vertical-growing/633929-pros-cons-going-vertical.html#post8823995