FSF GPL Licenses are the only open source licenses that protect the freedom of the creator and the users (the end user who actually uses the software as a product).
Most permissive licenses only seek to protect coder / author attribution. Programmers who use such license for their code don't mind if you use it in open or closed source project as long as they are credited for creating the code used.
But the goal behind FSF GPL is to ensure both author attribution and more importantly that the source code is always available to the user. So from a user perspective, FSF GPL license guarantees more freedom to a user because using a GPL license software means the user will always have access to the source code. And so they will always have the freedom to modify it.
As a programmer, GPL gives you the right to use or modify the GPL licensed code as you want. You have to share the modifications (make your source code open and available) only if you decide to distribute your application to others. That means if I buy the application from you, GPL says I as a user must also get the complete source code of the application (if I so desire).