Short answer: because that is how the developer sells support and updates. The key is a subscription. It was never permission to run the software.
If a plugin is prompting you right now and you want to know what to do about it, My plugin or theme is not activating is the article you want. This one explains why the prompt exists at all.
What a license key actually buys
Premium WordPress plugins are GPL licensed. The developer wrote the code, released it under a license that lets anybody use and redistribute it, then built a business around it. The key is how that business runs. It does three things:
- Authorizes update downloads from the developer’s server
- Gets you into their support desk
- Connects anything hosted on their side, such as template libraries and AI features
None of that is the code on your server. That code is already there, already running, and already yours to use on as many sites as you want.
Why almost everybody believes otherwise
Because the prompt is written to look like a gate. “Activate your license to get started.” “Your license is inactive.” Put that on a dashboard next to real warnings and it reads as broken.
Software trained us all for thirty years, too. You bought a program, you typed in a serial number, it ran. WordPress plugins do not work that way, and nothing in the interface tells you so.
So people see the notice, decide the plugin cannot possibly work yet, and go looking for the support form without ever opening the plugin. It is the most common thing customers write to us about. The plugin was working the whole time.
Keeping the two keys straight
Your 99Plugs update key goes in the 99Plugs Update Manager, and it is what makes updates arrive in your WordPress dashboard. Your keys are in the Update Keys section of your account dashboard on 99plugs.com.
A license key field inside a plugin is not connected to us and does not control your updates. Leave it empty.
The exception is the handful of plugins that hold their features behind an Activate button, where the field accepts any value. There, put your 99Plugs update key in so you only have one key to keep track of. CartFlows and WPML are the ones you are most likely to hit.
So is any of this legal?
Yes, and it is worth understanding rather than taking on faith. The GPL is the license the developer chose for their own code. Redistribution is what that license is for.
We wrote the whole explanation, including the parts that are genuinely arguable, in Are GPL WordPress sites legal, safe, and ethical?