Ham radio is a hobby and, like all hobbies, different people get different things out of it.
Some people enjoy restoring and operating antique equipment. Some people enjoy designing and building homebrew gear. Some people enjoy the challenge of working esoteric propagation modes, like bouncing signals off thunderstorms or aurorae or the moon, or relaying through the ISS or a handful of dedicated artificial satellites.
Some people emphasize the rock-bottom reliability of the technology. Two people with appropriate gear and knowledge can communicate with each other, and do so completely independently of any third-party infrastructure. This was a total superpower up until about 30 years ago, and remains appealing when thinking about emergency scenarios.
Some people are naturally competitive and will make a game of anything possible. These people have invented awards -- along with various pseudo-regulatory bodies to allocate same -- for things like making a radio contact on each of the seven continents, or for contacting the most people within a designated 48h period, or many other feats defined by various -- largely arbitrary -- rules.
POTA is one such set of rules. The "national/state level park" aspect is just to put some constraints on the competition. As others have pointed out, this is basically the same as niantic forcing you to get off your couch and walk down the block to the churchtower before you can catch a pokemon.
This requirement bleeds a little into the previous point, since it necessitates a portable rig (either human-carried or mounted to a vehicle of some sort). Such a rig would be more useful in a hypothetical emergency comms scenario than one that requires mains electricity, a power amp the size of a filing cabinet, and a 30m antenna bolted to the side of your house that just burned down.
But yeah, in the end it's basically Pokemon Go that requires custom hardware and uses national/state parks in lieu of defining their own map of pokespots.