They believe that every living being has come out of same supreme source. So by serving others indirectly means to serve the supreme source.
By offering Namaste to the other person, they remind to themselves and to the opposite person that they both are part of one supreme source, children of same consciousness and ready to do service to the opposite person.
The word Namaste means 'bowing head down'. This act indicates that they are no more than the other person whom they offered Namaste, as everyone have the same divine consciousness.
They even offer Namaste to the ants in their hearts. Because they believe that "if we have awareness we can see the divine consciousness in every living being". So they offer their respect to the divine consciousness in the others by this act.
Ultimately offering Namaste means to be in an awareness that
“I am a part of divine consciousness, I am the lover of divine consciousness, I am loved by divine consciousness, I am the servant of divine consciousness. So I always be like a well wishing instrument with the blend of humanity to the others, who are also a part of the divine consciousness".
This realization helps everyone to lead each and every moment of life with the raised conscious levels.
Offering respect to the others by Namaste is not just a body act or a lip movement. It is an awareness, felt from the heart.