You don't need to ask for their username.
You don't let them see what code looked like earlier in history.
You don't give write access to the repo.
It is much quicker to send a link.
The most useful scenario is when you want to give access to a potential employer. If you are talking to an HR, it might be awkward to ask for a username of the developer who is going to take a look at your code.
Also some people don't have an account on the platform you are using.
