The intent of copyleft is to ensure freedoms for the recipients of derivatives of your works. In software that means the users of forks. Copyleft restricts you to the same license (or a compatible one) to prevent you adding more restrictions. ““More permissive”” software licenses can be redistributed with the same license but often it’s a more restrictive license (e.g. MIT -> proprietary).
You can’t remove the license, say MIT, or add any restrictions to the code. You can combine it with other works and distribute the combined work under different terms, provided you still abide by the terms of the license (e.g. source distributions need to retain the license, binary distributions don’t).
Copyleft forces modified distributions to have the same terms as the copylefted software. If you have some MIT code and some GPL code and you combine it, the combined work is GPL. Likewise if you combine proprietary code and GPL, the combined work is GPL, and that’d regardless of how it’s distributed.
Permissive licenses protect the source as it was when it was combined into the other work. Copyleft licenses enforce distributed changes to the source are made available under the same license. Which one you want depends on what “freedoms” you want. Do you want the freedom to use the source however you want, including with proprietary code? Then you want a permissive license. Do you want to ensure that any changes to the code are always available? Then you want copyleft code.
The intent of copyleft is to ensure freedoms for the recipients of derivatives of your works. In software that means the users of forks. Copyleft restricts you to the same license (or a compatible one) to prevent you adding more restrictions. ““More permissive”” software licenses can be redistributed with the same license but often it’s a more restrictive license (e.g. MIT -> proprietary).
You can’t remove the license, say MIT, or add any restrictions to the code. You can combine it with other works and distribute the combined work under different terms, provided you still abide by the terms of the license (e.g. source distributions need to retain the license, binary distributions don’t).
Copyleft forces modified distributions to have the same terms as the copylefted software. If you have some MIT code and some GPL code and you combine it, the combined work is GPL. Likewise if you combine proprietary code and GPL, the combined work is GPL, and that’d regardless of how it’s distributed.
Permissive licenses protect the source as it was when it was combined into the other work. Copyleft licenses enforce distributed changes to the source are made available under the same license. Which one you want depends on what “freedoms” you want. Do you want the freedom to use the source however you want, including with proprietary code? Then you want a permissive license. Do you want to ensure that any changes to the code are always available? Then you want copyleft code.