Yoga in general is largely about reaching highest levels of consciousness; this happens also through purifying, and strengthening the body.
Cleanliness is very important to this; and there are many things such as good energy having to do with this.

Purifying the air and body is beneficial; one wishes to have clean air, this is important, strength and health of the body are helped by this.

Mantras are very helpful for purifying the body, which bring protection, strength, healing, and happiness;

These combined with strength and breath in clean and pure environment, is best.

Very slow movement, while breathing long breaths, is one of the most beneficial.

And Pranayam is wonderful… Not just sitting, there are several types as well. But sitting is generally thought of.