The Gershonites and the Merarites were all Levites and they all had a necessary place in the service of the Tabernacle and the nation of Israel, but their roles in that service were distinct and they weren't allowed to usurp each other's places. Each man had his assigned task. Although I'm sure there was some flexibility, overlap, and cross training within the clans, if anyone refused to do his job or took someone else's, the whole process of breaking and making camp was disrupted. One person could cause a chain reaction effecting everyone.

It's the same in our congregations today. We all have work to do and being jealous of each other's roles and missions will only cause chaos and reduce everyone's ability to function as a single body.
1 Corinthians 12:12-31