There is no reason to use anything higher than the lowest RPM for your power setting unless you cannot get the required manifold pressure or need more power.

It makes a large difference in fuel consumption. That is the main reason to use lower RPMs. Engine wear is also important too.

The reason the Arrow POH shows you the difference in fuel consumption at the various RPM settings and the Seminole doesn’t is really just a lack of standardization in the Piper POHs. There is a difference in fuel burn when you use the lower RPM settings in the Seminole. I just assume that the fuel burn in the POH for the Seminole is only achievable with the lowest RPM settings (I actually get about 2.1 gph less than published sometimes, but I always use the POH values or greater).

Also, the difference in power settings between the Arrow and the Seminole is because they have different engines.

If you want a simple way to set power, use units of power instead of only using the limited power settings published in the POH. Simply add up the RPM divided by 100 and the manifold pressure. For example, to produce 75% power at 3000 ft in the Seminole, the POH suggests a power setting of 25.6” and 2200 RPM. Simply add 25.6 and 22 to get a total power of 47.6 units of power. 100 RPM is equivalent to 1” of manifold. You can then use any combination of manifold and rpm settings as long as they add up to 47.6 to get 75% power with only one limitation. There is a limit of how low RPM you can go/how high manifold you can have. For the Seminole and Arrow, as long as the manifold pressure is no more than 4.5 units above the RPM settings, you will not damage the engine.

You want to operate at the highest manifold and lowest RPM you can to get the most efficient engine performance. In this case, a power setting of 26” and 2150 RPM is the best (26+21.5=47.5 units AND, 26-21.5=4.5 units).