Even assuming 100% solar rooftop coverage is the goal, given limited capacity of raw materials, labor, infrastructure would still necessitate prioritization of when to allocate those things to which places.
Then, once the top places are addressed, you can move onto the second tier of locations, then the third, etc...
This could be helpful if you're in gov't and have some control over a pilot neighborhood project. Or a developer that wants to include solar on some homes/businesses and wants to know where it makes the most sense.
You're right that this probably isn't too much better than qualitative reasoning about how sunny certain places are, but this is quantitative, so you can have a little more confidence in your qualitative assessment.
Aside from that grid operators buy power from producers. They don't plan future capacity more than 72 hours in advance. If you're a producer with expensive power you won't sell much. If you're a producer with cheap power you will sell a lot. It's already a functioning market. Solar is a very small part of it.