PAC files started making sense to me when I had a small group of domains that needed one route while everything else could use another. I was tired of switching settings manually, so I reviewed the automatic configuration options alongside the normal
firefox proxy setup and moved the routing logic into a PAC file. After that, Firefox selected the route based on the address instead of requiring me to change profiles during the session. It was especially useful for repeated testing because the same rules applied every time I restarted the browser. I still keep PAC logic simple because complicated rules become harder to debug, but for stable routing patterns it saved me a lot of repetitive switching.