So, have you ever just copied code off the internet? If you have, unless it has a license, laws such as WIPO Copyright Treaty and many others make that illegal. Legitimately, no permission from the author or license, no code allowance. The author(s) of code own the copyright automatically and unless you explicitly can do what you want to with it, it is a crime to copy the code.
So let's see what is on GitHub. I have found so many repositories with no license whatsoever. Have they ever considered this legal side? Have you ever considered this legal side? If you want repositories to be used, use a license.
So if you want your code to be used, do you write up a license as anyone can or do you use one of many already written ones that people trust? That is ultimately up to you but trust is important. Seriously, only copy code that you have either express permission for or that has a general purpose license. Do what you will but be warned. Final word, if it is on a website that states copyright, especially do not copy that. All as per one might be maybe or might not be maybe, maybe or maybe not, maybe.