Plain CSS has evolved considerably. Custom properties, nesting, cascade layers, and modern color functions can reduce the need for a preprocessor in some projects. Sass remains valuable when a project benefits from compile-time variables, reusable mixins, functions, loops, conditional logic, and an established module architecture.

Consider a website with 40 components that share spacing values, colors, breakpoints, and typography rules. Without a consistent system, the same values can become scattered across multiple stylesheets. Sass helps centralize those values and expose reusable abstractions.

Common use cases include:

• Design systems with shared tokens and component conventions.
• Large applications with multiple feature-specific stylesheets.
• Reusable responsive patterns and browser-compatible declarations.
• Theme generation and repetitive utility classes.
• Legacy projects that need structured styling without a complete rewrite.

However, Sass is not automatically necessary. A small application using modern CSS may be simpler without a preprocessing step. Choose Sass when its abstractions improve consistency and maintainability rather than introducing it solely because it is available.