SEO specialists without proxies are like without hands: checking rankings from one IP leads to captchas and bans. Proxies for SEO solve this — each search request looks like a new user from the needed region.
The main use is checking site rankings in search results. The search engine sees requests from different addresses and does not block. Second is parsing search results for competitor analysis. Third is working with regional search: proxies of the country where the site is promoted.
For SEO, address cleanliness matters: proxies for Google must not be burned in spam. Otherwise the search engine shows a captcha right away. Take a pool the seller verifies.
And remember: for ranking checks take proxies of exactly the region where you want to see results. Check a German site from German addresses.