What effort is "forfeited" (if you are talking about open source code)? If you use any GPL software, any modifications you make belongs to you until you choose to distribute the software with your modifications. Modifying a GPL software, for personal use, doesn't mean that you are obliged to make your modifications also open source. Moreover, GPL also means that you can never be denied access to the source code of a GPL software that is publicly distributed. This is because GPL protects a users "right to repair".

For example, consider the case of a software distributed under a permissive license like MIT or BSD. If you modify and re-distribute it, anyone else can further modify the software that you patched and improved but they are not obliged to release the new source code to you. In such a scenario, you are willing "forfeiting" your effort. With GPL, at least your right to access future source codes (of publicly distributed GPL software) cannot be denied to not just you, but any user of the software.