Sidelines serve multiple purposes in chess preparation. They can be used as surprise weapons against well-prepared opponents who expect the main line, as practical choices that avoid heavy theory, or as strategically sound alternatives that lead to comfortable positions with less memorization required.
Some sidelines are objectively as good as the main line but simply less popular. Others involve a small concession in exchange for practical benefits like simpler positions, less theory to learn, or types of positions that favor your playing style. A few sidelines are dubious but can be effective against unprepared opponents.
The value of sidelines increases at amateur levels where opponents are less likely to know the precise responses. A well-prepared sideline can force opponents to think for themselves from an early stage, negating any preparation advantage. Many strong players maintain a repertoire of sidelines alongside their main-line choices.