The GPL does put restriction on use. If you make modification to the code and do not share those modifications, you are constrained from using the code you modified.

And those constraints are for political reasons. The Four Freedoms are a political ideology about how software, developers, and users should morally interact, and the GPL exists as a legal tool to support this political ideology.

Edit: clarification - by "use" here, I'm including "bundle it into a black box of hardware and sell that hardware."