Short answer: Everything!
Long answer: Due to Apple's unreasonable company politics, you cannot write into /usr/ (and many other directories) as this goes against their System Integrity Protection which:
[...] is a security technology designed to help prevent potentially malicious software from modifying protected files and folders on your Mac. System Integrity Protection restricts the root user account and limits the actions that the root user can perform on protected parts of the Mac operating system. (source)
This is absolutely bullshit when you consider the fact that Apple actually lets you.. kinda? disable this stupid protection without the option to disable it actually working!
To disable it you have to find your way into the RCM (Recovery Mode) and then type csrutil disable, bla bla bla.. Though, it doesn't work, because the next time you want to write into these protected parts, macOS will throw you another error which neither I nor a friend of mine could fix so far.