This is a trick for presidents and vice presidents.
When it comes to matching battles, the game follows an algorithm to balance the teams as much as possible.
After countless battles we have discovered that when strong cities are put as members, they are taken into account less than if they were seniors or vice presidents.
Therefore, the strongest cities, (not only in battle experience, but in War level or even in Contest of Mayors level), should have the member rank. That way, the algorithm takes them less into consideration and, therefore, the opponent will be a little weaker.
Important note: of course, at some points, this trick stops working. It is usually in the arena 7 of War and from 5000 points.