It prevents people from showing up randomly and ringing doorbells. The realtor can control the process, inexperienced and stressed sellers are not confronted with strangers 'negotiating'. Mostly it's in the interest of the realtor that their position as a middleman is accepted.
Realtors in the US typically have an exclusive contract with the seller and the seller must pay the commission however the house is sold. Without a contract like that (seems common in Europe), a buyer can bypass the realtor and work directly with the seller. The listing agents hide the address to try to prevent that from happening.
