manifest.json contains list of your dependencies(the packages you have used for your project). so if You clone your repository on a different machine, unity uses this list to download the required packages.

short answer: manifest.json is important.

manifest.json contains list of your dependencies(the packages you have used for your project). so if You clone your repository on a different machine, unity uses this list to download the required packages.

short answer: You should not put Packages in your .gitignore. It should be committed along with the rest of your project files.