Why should be useful take a look on already discovered XSS?

Because very often a single XSS vulnerability maybe replicated over the whole application or in a single part of it. apart from this it very interesting to understand methodology used by others to find vulnerability. So take a look at these lists and learn from already discovered XSS vulnerabilities:

Image source: https://www.kitploit.com/2018/05/xss-payload-list-cross-site-scripting.html