Let me give a pretty nice comparison between both of them. Have you ever played chess? If you haven't then let me explain while playing chess on board in front of real opponent you have to sit in front of him eye to eye facing each other like you are fighting. Now, this things is a great psychological factor in thinking. Most of the players get pressurized on certain important moments and make many wrong moves while some are good at reading body languages so become cautious if their opponent sees happy. So strategies do change with age & mentality of your opponent while in online chess it's more or less like to play against a computer all you can see is 64 squares and 32 pieces.

Virtual world has given us access to doing everything from distances right at the comfort of our homes. But one thing it can never match is the feel of the reality and aura which a person creates with his physical presence. Playing against Magnus Carlsen(No. 1 chess player) on computer you won't feel much. But playing in reality against him and you wouldn't be able to get your head on board. This is true for almost every sport.