Just like a regular individual RRSP, the money in a Group RRSP is tax-deferred and intended for retirement. The main advantages are:
Entirely voluntary enrolment
Employer matching is available, but not required
Payroll deduction of contributions means immediate tax advantages
Access to lower-than-retail investment fund fees
Very low administrative involvement for employers
Employer contributions to a Group RRSP are deductible as a salary expense to the employer, but are taxable as income on the employees T4.