The simplest thing you can do with Power Armor Frames is to add a Fusion Core to them, and fast travel to an area. Fast travel does not drain Fusion Cores, which allows for a few things.
If you are encumbered, Power Armor adds Strength which in turn grants a higher carry limit. This can be enough to take you out of encumbered state, allowing travel to an area where you can dump all the looted items.
Placing a Power Armor Frame in each of your settlements that have Power Armor Stations, means you can remove a Core from one set of Armor, and travel to the next area with an armor waiting, and power that armor up with the same core. having specialized armors in each area might be a nice idea for some people.
Power Armor resists radiation by a high amount, making underwater travel safer. While not completely safe, it's a good option to have a specialized set just for underwater exploration.
Having Power Armors in each area means you can use the station to craft. without an armor nearby, you do not get the option to craft, even if you have power armor parts on you or in storage.
There's not much you can do with multiple frames, but even a coreless frame makes for a nice stand for the armor. Having a whole line up of robot suits just as decoration and collection purposes is pretty cool.
Edit: As pointed out earlier, settlers might be able enter frames to defend the settlement.