Are child themes essential? Kind of forgot to do one until now

I kind of forgot about child themes until a couple of days ago, I vaguely remember something about them from building a joomla site 5 years ago.

Do i really need one? The problem is I don't know which parts of the site I'm building I've already changed which will be affected.

If I need one, what's the easiest way to do this without ruining any custom things I've already changed?