If you're building a site or something, why would you use WeatherKit's REST API rather than using something like OpenWeatherMap?
If you're a weather app, being able to tell your users "it will start raining in 35 minutes but pass in 55 minutes" is a huge value add over "80% chance of rain in the next hour". I found it to be quite accurate down to the 5-10 minute increment when I used to live in an area with frequent summer thunderstorms.
b) OpenWeatherMap limits not just by total API calls but by API calls per second.