Why build calculators instead of another comparison table?
Because the question a buyer has is almost never “which panel is cheapest”. It is “what does my order cost”, and the answer changes with quantity in ways a per-1,000 rate does not show. A panel with the lowest rate on the page is irrelevant if its minimum order is ten times what you need, and that is exactly the kind of constraint a static table flattens away. The price calculator exists to put the limits back in.
The same principle governs where the arithmetic runs. Price calculations happen on the server, so every result has a URL you can share and a search engine can read. Everything that involves a number you would not want us to see (your margins, your account figures, your API key) happens in your browser and stays there.