One of the advantages as stated here before is, that when joining a game that's already in progress, it would make it possible to have a persistent world(that is until the server crashes ;). This would mean that your actions actually would have consequences and would influence the rest of the game, hours or even days later. Unlike now, after the map is played, its all over and you restart with a new map.
OFP 1 online is more waiting then playing.