Test the gate, not the code path
Geo restriction logic is easy to unit test and easy to get wrong in production, because the production answer depends on a geolocation database you do not control. Connecting from the actual country is the only test that exercises the real lookup.
CDN and latency behaviour is regional
Which edge serves a user, and how quickly, depends on where they connect from. Testing performance from one location tells you about that location.
Consent and compliance banners key off region
Privacy prompts appear or do not depending on jurisdiction. Verifying that the right one shows in the right place means arriving from that place.
Mobile behaves differently again
Carrier networks apply their own routing, and some apps treat carrier addresses differently from broadband. If your users are mostly on phones, test from a mobile IP.
What to buy
Residential for regional web testing, mobile if your users are primarily on phones. Small metered amounts are usually enough for QA.
Common questions
Why test an app through a proxy at all?
Because a large amount of app behaviour is decided by where the request comes from. Pricing, available features, content licensing, language defaults and legal notices all vary by country, and none of that is exercised by testing from your office. A proxy lets one machine stand in for a user in each market you ship to.
Which pool should I test from?
Mobile if the app is a mobile app and the behaviour you are testing depends on carrier conditions. Residential for anything where the app should treat you as an ordinary consumer. Datacenter is fine for testing that an endpoint answers at all from a given country, which is a lot of routine checking.
Can I test how the app behaves on a slow connection?
Not with a proxy alone. A proxy changes the path and therefore the latency, but it is not a network conditioner and cannot reliably reproduce a specific bandwidth or packet loss. Use the throttling tools in your test harness for that, and use the proxy for the question it does answer, which is where the request appears to originate.