About six months ago I was reviewing a pull request that added a new "developer utilities" page to our app. The implementation sent user-submitted text to a server-side API for processing. The submitted data included snippets of application code from our codebase.
I asked the author: "What happens if someone pastes a password into this?" They hadn't considered it. The data went over the wire, landed in a server, was logged in three places.
This is the moment I started preferring browser-based tools.
No data leaves the machine. When you use a JSON formatter that runs client-side, your input never touches a network interface. There's no request carrying your source code or proprietary data to a remote server.
What makes it possible. Web Crypto API does SHA-256 in-browser. Canvas handles image processing. TextEncoder/TextDecoder covers encoding. The File API reads files without uploading.
When it matters most. You're formatting a config file that contains API keys. You're testing regex against production log data with PII. You're on an air-gapped workstation. In all three cases, network-based tools are a liability.
The offline bonus. Many browser tools work without an internet connection once loaded. If the computation runs locally, connectivity isn't a dependency.