A developer shares a useful program so others can inspect, modify, and redistribute it. They may also want those freedoms to remain available when someone distributes a modified version. Copyleft makes that continuity part of the license conditions.
Copyleft uses copyright rather than abandoning it: permission is granted on terms that preserve specified freedoms downstream. It differs from a permissive license that may allow modified distributions under more restrictive terms.
Sharing forward
Imagine a group adapting a program for a local association. If it distributes covered modifications under a copyleft license, the relevant conditions govern what recipients must receive and may do. The license specifies which modifications and distributions those conditions cover.
Private editing and distributing a modified version can trigger different obligations. The relevant license determines when recipients must receive the covered permissions and materials; copyleft does not impose one blanket publication rule on every private edit.
Copyleft can support a continuing pool of reusable work by limiting how recipients may close off covered redistributions. But it does not supply maintainers, resolve disagreements over the project's direction, or ensure contributions return to the original team. It preserves legal permissions; the social organization that makes those permissions useful still has to be built.