Pro plan will allow you to benefit from more data centers, that reduces latency. You also get some CDN priority over free plan, better TCP connections, more websockets connections, get access to polish that will optimize your images, Enhanced HTTP/2 Prioritization, and more… Argo can help with TTFB and connections issues.
Argo Smart Routing gives you a new set of IP addresses. From my experience, these IP addresses are routed similarly to those given to Enterprise zones. This means you will be able to leverage most, if not all, of Cloudflare’s network.
Does Cloudflare provides another unique IP for the pro plans? i checked, the free plan server ip and location is Amsterdam, North Holland, Netherlands (NL) IP is something like 18.104.22.168. there is millions of sites hosted.