Search engine optimization (SEO) is a massive industry, driving most online businesses. Why you need them for SEO? Well, you can research competitors through an SEO proxy service, you can scrape SEO data, etc. The thing why you need proxies is that most of the SEO tasks are completed with automatic tools sending requests over and over again. When you do that with 1 IP address it gets detected very easily and it ends in the IP being blocked. When you use proxies, you can avoid that since you will be constantly provided with the new IP (when you use rotating proxies) so it not only will be difficult to detect you but also you will avoid IP bans. There are several great proxy providers for SEO, I would recommend Smartproxy or GeoSurf in this case.