Answer:
It is an honorary title bestowed on one by the queen (or king) for meritorious service of a high order. It began in the middle ages, when those men who protected the king and kingdom (the king's "army" in effect) were honored for their bravery and service. Knights earned the title "Sir" (a big, big deal back then) and thereafter had certain perks, including being the only people in the kingdom besides the queen who were allowed to sit down in the presence of the king.