There was a time in 16th century Holland when there was plenty of herb (they grew it to make sails) but this new thing, tobacco, was scarce. So when then could get it, they would conserve it by puttings small amounts of tobacco in their spliffs. They been doin' it ever since.

Mixing them is used sometimes in order to be able to smoke herb without getting sleepy or hungry.
It must be remembered that tobacco, tea, coffee, sugar where important in Europe, especially England, because the people were so hungry and these things give (false) energy and suppress hunger.

In India, all Sadhus smoke tobacco with herb or hashish in their chillums. Sometimes they add a couple drops of cobra venom to warm themselves up if the weather is cold (would kill most people).
They spend years doing yoga and breathing exercises and the tobacco don't seem to harm them. You ever see somebody inhale for 3 minutes, hold it for 6 and exhale for 3? Beyond the lungs there is something called cellular breathing, and to learn how to link those two is part of the ancient culture. But they don't siddung all day in front of the television or computer.