Why should I use .gitignore?

It hides the file from git status. Especially with a lot of generated files you really don't want them to show up in there all the time. It also prevents you from accidentally adding them e.g. when doing git add somefolder.

The purpose of gitignore files is to ensure that certain files not tracked by git remain untracked.

You'll generally find that adding each and every file one by one can become tedious when your project becomes bigger.

In Java, you may end up with many .class files that you wouldn't want to add to your repository. Similarly, you may end up with many .o files in C, or .pyc in Python (for example).

Having patterns like *.class, *.o, *.pyc in your .gitignore files allows you to ignore those files once and for all. Subsequent git status runs (or similar if you're using a GUI) will not tell you these are new, untracked files, thereby letting you focus on the files that are really new and noticeably untracked.

This can also be useful if you add an entire directory (e.g. git add myproject): this lets you ignore a category of files altogether.