Games, movies, television programs, music, and even websites, are all designed to be seen or heard by us; The Target Customer. What kind of music do you like? Rock, Hip Hop, D&B? Any music that fits into those categories was designed to be in that category. Although artists don't always like the idea of being "pigeonholed" into one category, it is unavoidably necessary to decide beforehand what that category is going to be.
Gamers are seperated into three categories; "Objective based", "Competition based", "Personal Challenge" (i.e. breaking your previous high score). Gamers have been even further divided into "hardcore" gamers and "casual" gamers.
Objective based gamers like to see progression. They will often try to complete every achievment in a game even after they have played through the storyline. Or they might try to "max out" their characters by leveling up. These players tend to like open world, sandbox, and RPG style games, in which a certain amount of choice is given to the player to complete those objectives in their desired order.
Competition based gamers love to go head to head with other players. Often the only reason they will play through the storyline is to get "good enough" at the game to feel confident going up against other players. Multiplayer capabilities are a must for this demographic. They want to see public leaderboards and killboards with their names on them, and might set their sights on knocking the person off the top.
Personal Challenge based gamers like to best their own high scores. They like to challenge themselves rather than other people. Arcade games and Strategy games are popular amongst these gamers.
Even if you feel like you may enjoy a little from all three columns, as I do, there is still one group to which, you predominantly belong.
Why does this matter?
If you are designing a game, you have to consider the possibility that the ONLY ones who will buy it, are the ones you designed it for. Which also means; if you designed a game for yourself, you might be the only one who buys it. :)
Decide who your target customer is before you ever start brainstorming an idea, and your pre-production phase will go a lot easier. :)