Why does returning an incumbent to office allow constituents to take advantage of the seniority system?

Senior members gain privileges

Explanation:

When an incumbent is re-elected he gains special privileges that new members do not have. The longer a member serves the more power they acquire. They can choose which committees they are assigned to, can become heads of committees, and more. Seniority, especially in the House of Representatives, is powerful and gives constituents, whom they represent, more of a voice in the Legislative Branch.