In many articles on this site, you’ll see me refer to the importance of development. And while hammering this point home again and again often gets new players to stop trying to play chess with a single piece (particularly the queen), it’s still common to see improving players to neglect the true importance of developing all of their pieces.
Here’s a helpful way of thinking about exactly why using all of your pieces is important. At the beginning of the game, none of your pieces are doing much of anything to influence the board or put any pressure on your opponent. In a sense, they’re sitting on the bench, waiting for their coach (that’s you) to put them into the game. Like any good coach, you will want to get your players on the field, rather than allow them to sit on the sidelines.
When one player fails to do this while the other player does develop their knights, bishops, rooks and queen, this analogy becomes even easier to understand: it’s essentially allowing your opponent to have an entire team on the field, while you try to play with a single star player (usually, for new players, their queen) or half of a team. Effectively, this gives your opponent a material advantage even if your pieces are technically on the board. Even LeBron James, Peyton Manning and Lionel Messi can’t play by themselves against a team of players working together! Only by using your entire team in concert will you be able to compete against strong players who regularly use their full army in every game.