Many countries adopt the policy of stunning cattle before killing them as it is supposedly more humane. But Muslims and Jews have religious prohibitions against consuming the blood of animals and so they oppose stunning and instead require that in order for the food to be certified as kosher or halal, that the animals must not be stunned and their blood must run freely when killed, presumable to drain the meat of all blood.
The idea that animals lose all their blood even when slaughtered this way does not make sense but scientists have tested this idea and find that stunning does not affect the amount of blood lost.
Of course, this is not going to change the minds of orthodox religious believers. They will insist on doing things the same way that their ancient ancestors prescribed, modern science and knowledge be damned. All that will change in the light of new evidence are their reasons for maintaining the practice.