[kmacleod]

Permissibly licensed software does not require you to provide sources or the changes to it.

Copyleft licenses require the source, including changes, to be provided.

The original purpose of copyleft is roughly the "right to repair". When you buy a product: you receive the necessary source, build instructions, and tools in some cases, to update and replace the software you received. The use of copyleft software passes those requirements on through proprietary users.

Note that one can still build a proprietary product as long as all the copyleft pass-through software can be modified and replaced on the product.