Under 61.133(b), a commercial pilot who does not hold an instrument rating is restricted: they may not carry passengers for hire on cross-country flights of more than 50 NM, or at night. This limitation appears as a limitation on the certificate.
The rationale is safety — carrying paying passengers far from home or after dark demands instrument competency. Earning the instrument rating removes the limitation. For this reason most commercial training assumes the instrument rating is held first.