The main role they exist for is for long-term things that need steady power flow and don’t have access to plugins. Besides aerospace and medical implants, military use is the main one. I’d say a lot of survivor stuff does come down to, effectively, military use. (Edit, however as nameless survivor points out, we’re not going to be able to find you specs on military plutonium batteries online)
What are plutonium cells meant to be if not plutonium batteries? A big part of where I started with all this is the description of a plutonium cell in-game, “A nuclear-powered battery. Used to charge advanced and rare electronics.” and a big part of the confusion in this thread stems from me thinking batteries are often used to power lights, and you saying this one can’t power anything stronger than a nightlight.
Anyway. Back to the fun historical stuff and speculative engineering. It’s hard to get exact specs for the thermopile outside of the pacemaker, but in the seventies, plutonium BV cells somewhere in the 40-80 gram range containing less than .2g of plutonium produced 3-400 uW of power. A comparable cell scaled up to plutonium cells in game would produce somewhere around 30-40 mW. If the plutonium cells in game are alpha voltaic, or have more efficient energy capture than a cell developed in the seventies, or require less shielding, they could easily be much, much more efficient. In fact, it would be puzzling if they weren’t. For one thing if they’re using plutonium it probably means they’re using alpha capture rather than beta, which is higher energy but apparently wasn’t feasible in the seventies (the original papers just say they wish it could be. Alphavoltaic cells do exist now but I haven’t found much about them yet). I’m spitballing, and I’m no engineer, but I’d assume a plutonium battery by the tech level of cdda could produce a hundred times the energy of seventies level tech, considering what 2018 BV cells using different isotopes can produce.