Not sure the specific MySQL thinking, but FKs in a large data model help new people understand or confirm relationships, and also make importing your data model into an ERD tool for documenting to others much easier. FKs which enforce relational integrity are always useful against bugs. Some query planners (e.g Redshift) even use FK relationships to optimize query plans even when there aren’t indexes nor constraints provided by FKs.
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Not sure the specific MySQL thinking, but FKs in a large data model help new people understand or confirm relationships, and also make importing your data model into an ERD tool for documenting to others much easier. FKs which enforce relational integrity are always useful against bugs. Some query planners (e.g Redshift) even use FK relationships to optimize query plans even when there aren’t indexes nor constraints provided by FKs.