When conserving food in jar, why is the jar turned on its head when cooling?
Generally turning a jar upside down after filling is a alternative to a proper hot-water bath, not an addition to it.  This is known as the inversion method, and is used by many cooks of jams, jellies, mustards, and other very-low-risk canned foods.

The idea of inversion canning is that, by forcing the air in the jar through the hot liquid, you can kill and bacteria in the air (and on the inside of the lid).  It is not considered safe by experts, because there are numerous ways that bacteria can survive the process.

If you are doing "proper" canning, that is heating up the lidded jar to 85C or above, the sealing, inverting the jars is in fact a bad idea as it could result in liquid preventing the jar lid from sealing properly. The Ball jar corporation says “Do not invert. move or store jars while cooling, as this may cause seal failure.” At the least, it's unnecessary.