Some targets key their rules on the network an address belongs to, not only on the country. Cloudflare's rate limiting rules, for one, can count by ASN on their Enterprise tier. A line pinned to CenturyLink looks like one consistent kind of visitor to a rule like that: a CenturyLink customer.
The other reason is testing. A page, an ad or a price shown to CenturyLink customers can be checked from a CenturyLink address, from the countries listed above, without asking anyone to run a browser for you.
A pin does not change what your client sends. Cookies, headers and the TLS handshake are still yours, and a target that keys on those needs a real browser profile behind the line.
Keep to public data and to the target's terms. The ISP targeting page lists every carrier we can pin.