Short answer: Composite rating gives the employer stable, simple tier rates that don’t shift as employees age during the year, which eases payroll and budgeting, and can favor groups with an older workforce.
The appeal of composite rating is predictability and simplicity. Because rates are set by tier (employee-only, employee+family, etc.) rather than by each person’s age, payroll deductions stay consistent and the employer’s budgeting is easier. It can also be advantageous for groups with an older average age, since younger and older employees pay the same blended rate. The trade-off is that a young workforce might pay less under straight age rating.