IPv6 provides a vastly larger address space than IPv4, allowing proxy providers to offer large numbers of addresses at a much lower price per IP.
This makes IPv6 proxies useful when a task needs many addresses and both the target and the customer’s software fully support IPv6.
IPv6 describes the internet protocol version, not the network origin. Most commercial IPv6 proxy services still use datacenter networks, so an IPv6 address is not automatically residential, mobile, or harder to identify.