Negative lookaheads (and lookaround constructs in general) are an awesome feature in PCRE. But what if you are using a non-PCRE engine?
When scanning with Metasploit Pro, your victim can counter with a XSS payload, and even take over your machine. Never trust your victim!
Sometimes (expecially during CTFs) I need to display the result of a REALLY specific HTTP request that I made with cURL into Chromium.
Sometimes, you need to write a source file and “compile” (as in “run a generic script on it”) each time you edit it, just to see the final result.
a.k.a. “How the sausage gets made”