Licenses are necessary for reuse due to copyright, which automatically protects original works of authorship such as code. Without a license, default copyright protections apply which can prevent people from copying your code or making any derivatives.

Similar to how it's technically possible to reuse any image you find online, however, it may be copyright infringement unless you get permission from its creator. Open source software licenses essentially grant anyone permission to reuse your code under certain conditions. They also have disclaimers which are important. So if somehow blocktivity causes someone financial loss, you would not be responsible.

Take a look at https://choosealicense.com/no-permission/ for more information the problem of not having a license.