Proxies for ad verification
An ad bought for one region is hard to check from another. Viewing the placement from a local home or mobile connection shows what your audience is actually served.
Look from where the ad is sold
Set the country, and for residential the state and city, then load the page the ad should run on.
Check mobile placements on mobile networks
Many placements are sold for mobile only. Carrier IPs let you confirm those from the right kind of connection.
Keep sessions short
A new IP per check avoids being served a capped or personalised version of the page.