IPv6 is chosen when many addresses are needed, and the budget is limited. For the same money, you get many times more addresses than in the IPv4 format.
Typical tasks
Mass collection of public data, where the number of addresses is important.
Working in a large number of threads simultaneously.
Tasks where addresses are quickly consumed and many are needed.
Testing websites and applications from different addresses.
Any large-scale operations where the price per address is crucial.
What to check before purchasing
Before taking IPv6, make sure that the website you need supports this standard. If there is no support, working with it through IPv6 proxies will not work. The check takes less than a minute and is described in the article How to check if a website supports IPv6.
When to consider another format
If the target website does not support IPv6 — IPv4 or another format is needed: they work with all resources.
If the website is picky about data center addresses — ISP or Residential.
If the address should change on a schedule or by link — Mobile.
If a wide coverage by countries and cities is needed — Residential.
Detailed comparison — in the article How IPv6 differs from other proxy formats.