Datacenter addresses are registered to hosting providers and cloud networks. They sit on fast lines, so they carry a lot of data quickly and cost little to run. That is why datacenter proxies can be sold by time with no traffic limit, instead of by the gigabyte.
The drawback is that a website can look the address up and see it belongs to a server, not a person. Sites with strict bot filtering challenge or block these addresses. Datacenter proxies fit targets that accept any visitor: APIs, open data, large downloads. For stricter sites, a residential or mobile proxy is the next step up.