The point is to allow bots that play nice and don't claim to be real browsers, as those can be identified in logs and blocked or rate limited fairly. But bad faith bots can be undistinguishable from browsers, so everyone else gets a PoW challenge to make their endeavour slower and expensive.

At least that's the spirit, of course someone will eventually just use random strings as user agents, but then again this is all a tragedy of the commons anyway.