Properly designed SQL relationships are crucial for building efficient, maintainable databases. The five key relationship types - one-to-one, one-to-many, many-to-one, many-to-many, and self-referential - enable modeling complex real-world scenarios while maintaining data integrity. Well-designed relationships reduce data redundancy, improve query performance, and enforce referential integrity, keeping data consistent across tables. They also provide flexibility and scalability, allowing for easy extension and modification as the application grows. By following best practices, such as indexing and constraints, developers can build robust database structures that perform well now and scale effectively in the future. Platforms like Strapi v5 offer tools and guidance to manage relationships without raw SQL, making it easier to create complex data models.