The most common case is brand abuse: when someone copies your company's name, logo, and design to set up a fake site, checking it from your corporate IP shows a clean version while the phishing form served to real users only appears once you look like an ordinary consumer — see our brand protection proxy page for that workflow in depth.
Phishing pages are also often region-specific — a fake bank page might only show its card-details form to visitors from the target country and redirect everyone else to a harmless page. Without checking from different countries, you can't fully tell what the page is actually doing or who it's targeting.
IP reputation matters when gathering threat intelligence too: requests from a known security vendor's IP range are often noticed and blocked, or served different content, by malicious infrastructure. Researching from a wide, ordinary-looking IP pool keeps the data you observe representative of the real attack infrastructure.