It’s not just the actually copying of source code that’s protected, but also the structure, sequence, and organization of the computer software. This stops third parties from being able to just change the source code slightly and end up with the same outcome as with your software and not count that as copyright infringement.
If you register your software within the first three months of publication, you could find yourself receiving between $750 and $30,000 for each of your works if they are indeed infringed upon. Sums of as much as $150,000 aren’t unusual in the cases of willful infringement. As well as the option of fining the infringers, there are other remedies such as temporary and final injunction relief to make sure no other infringement can happen in the future. The costs and attorneys’ fees are sometimes enough to make the infringer come to a settlement more quickly before the costs rise and this means you won’t have the hassle of going through the courts.
Note that copyright protection is only valid for the version of the software that you registered, so every time you update the software you need to register the new version with the Copyright Office.