Like to sort of give you an example, when my friend and I both had Street Fighter 3rd Strike, I got to play online and with friends and he didn't. While he played the single player, he actually got really, really good at the core game itself; he nailed parrying, the timing of characters moves, what characters were good against others, etc etc.
But when I played him, even though he arguably had a better working knowledge of the game than I did (and I'm no slouch), I've been playing against other people, so therefore I knew all about tactics such as mix ups, mind games, cross ups, footsies, etc etc, tactics that aren't applicable against the computer. I whooped his ass shortly after. Sooner than later he caught up with playing with other people and now he's much better than he ever was before, now that he plays against real people who formulate strategies, adapt to new situations, and have the advantage of psychology (which is a big part of fighting games, fucking with the other guy's head)