The “seniority system” allocates authority in the House and Senate by seniority — I.E the number of times the person has been re-elected. In abstract, this system is numeric and impartial, and it is seen as an improvement on the cronyism that dominated in the 19th century. However it now biases Congress away from effective problem-solving.
Seniority-based promotions are problematic in any organization. The Congressional seniority system is thought to contribute to the partisan polarization in Congress. For example, moderates from competitive districts are not typically reelected decade after decade. Consequently, the senior leadership in Congress is inherently selected for partisan extremism. Seniority may also contribute to the recent growth in ethical lapses in Congress by increasingly safe Representatives. Conversely, more senior members of Congress have a better command of the institution, as shown (for example) by the positive correlation of the seniority of a bill’s sponsor and the average seniority of its co-sponsors.
There have been and are many reform efforts, which have a history of producing zero or mixed results. Ironically, the greatest success of the mid-20th century reforms has been the creation of a more polarized Congress, as was intended.
Gerrymandering helps members become more senior, because the results are baked in and their elections are safer than they otherwise would be. In 2012, the chairs of the most powerful House committees had been in Congress an average of 28 years each. These powerful members of Congress are highly beneficial to their districts and their states. There are multiple reasons for state legislatures to control the results of House elections through Gerrymandering, including the advantages conferred on their state by the seniority system.