Most experienced players use custom races because:
- These are usually better than the pre-defined races.
- Experienced players all have their own opinions and playing styles, and want to demonstrate the superiority of their ideas.
- In single-player games against AI opponents, some experienced players deliberately use weak races to to give themselves a more challenging task.
You can lose a game by bad race design. Good race design is unlikely to win the game for you against a human opponent, but you need at a good race design in order to have a reasonable chance of success against a competent player.
Your race design and playing style have to fit well together - for example if you choose a blitz race (similar to rushing races in real-time strategy games) you need to attack as soon as possible rather trying to expand by colonization, because a production race (one whose main strength is industrial production) will colonize and develop its colonies faster than yours. And different race designs are at their best in different galaxy sizes and / or with different numbers of opponents - blitz races are at their best in small or crowded galaxies; production races are usually rather weak in early-game research, so they need less crowded conditions, but they become monsters if given time to develop their economies.
Because race design is so complex and important, the details are split over a few pages - see the navigation links on this page or the Table of Contents.