The obvious difference between Correspondence Chess and OTB Chess is that when you play over the board you must do your calculations in your mind's eye. You have a clock running down and you must complete the game in the time allowed.
There is no such thing as playing out all possible scenarios before choosing what you believe to be the optimum move. There is no luxury of days on end before making a decision.
When you play OTB, you play one game at a time, as these games are played in real time. Correspondence Chess games, whether played through the mail or on a server, can be played in batches. Players can have more than a hundred games going at a time.