Why this one needs a probe
A browser can request another site, but the same-origin policy hides almost everything useful: you cannot read the status code, the headers or the certificate of a cross-origin response. A probe has no such restriction, so it reports the whole picture.
When you’d reach for this
When a site is slow and you need to know which stage is to blame: name resolution, the TCP handshake, the TLS negotiation, or the server thinking. It also shows whether a certificate is valid and when it expires.
Learn the idea behind the tool
This tool is the fast answer; the lesson is the understanding. Start with HTTP & TLS.