In the current system, some people's votes, those of the least populous states, count for far, far more than some other people's votes (those in the most populous states).
Plus, today, if you live in a state that is overwhelmingly in favor of one party, your preference for the other party doesn't count in the slightest. If you're a blue Texan or a red New Yorker, there is 0 point to you even going to the polls. In a national voting system, your votes would truly matter as well.