Paste one JSON file on the left and the other on the right. Every line that differs is highlighted, and inside an edited line the exact value that changed is picked out — so a single altered digit in a long config file is impossible to miss.
JSON does not care about spacing, but a text comparison does. If one file is on a single line and the other is indented, everything will look changed. Pretty-print both first (most editors do this with one command, and python3 -m json.tool file.json works from a terminal), then compare. Sorting the keys the same way in both files helps too.
Checking what changed between two API responses, spotting the one setting that differs between a staging and a production config, reviewing a package-lock or composer.lock before committing it, and confirming an export actually matches the original.
Yes. The comparison runs inside your own browser using JavaScript. Your JSON is never uploaded, never stored, and never logged. You can disconnect from the internet after the page loads and it still works — which matters when the file has API keys or customer data in it.