A developer needs to know how a site opens in Brazil, Japan or Germany. Testing proxies give this: the request goes through an address of the needed country — and you see the real picture.
What is checked: availability (does the site block the region?), load speed, localization correctness, CDN behavior. Proxies are essential for QA teams.
For test automation an API is needed: the script itself requests proxies of different countries and runs checks. This is a standard in modern teams.
Stability matters more than speed here: a test that fails because of a proxy is a false alarm in the report.