The purpose of this template is to allow users to specify that they don't mind their older contributions being copied to other sites, such as Wikipedia.
Why is this needed?
Although the CC BY-SA 4.0 License applies to all contributions made on or after January 5, 2016, contributions made before this date are not covered. Wikis are generally known to be places where users can share information freely, but we can't ignore the rights of the original authors. While very unlikely, it's possible that a user wanted or intended information to remain on this wiki, and not be shared on another site.
Think of it like this: let's say your cell phone plan includes unlimited data. Your carrier has decided to retire the unlimited data plan. Would you appreciate it if you were pushed onto a plan with 2 GB of shared data instead? Or would you rather the carrier let you keep that unlimited data plan?