They aren't a replacement for chemical batteries, more a complement. They're good for battery topping. Basically even when a device is completely off, its battery is still slowly losing power. This loss can be quite small, but if you leave a phone or something in a box for weeks, it'll be dead. This is not really an issue for consumer electronics, just charge it again. However there are applications where you want battery powered electronic devices that are ready to go when you need them and stored away from the electrical grid, like an emergency phone in a box on a remote hiking trail, or equipment in an emergency kit stored in a lifeboat, or a medical device in a first aid kit. By adding a nuclear battery, it can pump power in at roughly the same rate the battery would deplete itself when off, so even after years of sitting unused, your device still has a full battery the moment you turn it on.

Besides emergencies, there are also situations where the device has a low duty cycle, and thus its average power requirements are very low. For example a remote sensor that only activates for a few seconds per day may consume thousands of times more power for those few seconds than a nuclear battery could put out, but the rest of the time it could be recharging such that it has as much energy available the next time it turns on.

For most of the global, A solar cell can do the same thing. It's really only in the most northern and southern regions that a solar panel becomes ineffective.