Xenophanes basically figured out the basics of modern epistemology (how we can be sure that our knowledge is reliable). He put down some basics of how to approach knowledge and science that people didn’t fully understand until the 20th century.
He also dabbled in politics and arrived at the conclusion that the values that we should hold dear are individual freedom, tolerance and the continued progress of science and civilization – he said this in 500 BC when most people were pretty intolerant and had generally had no real knowledge of science – most people thought that things in the natural world (such as thunder and fire) was caused by gods.
But the main thing about Xenophanes is that he figured out the thing about knowledge. Basically, we can’t prove things by finding arguments for them. You can always find a thousand good reasons for why something you want to be true is true. So rather, we need to look at the arguments that we can find against things, not for them. It’s the things where we can’t find any good arguments against them, which are true.
This isn’t difficult to understand, once you’ve flipped that switch in your head, but most people have just never done so. People still go around listing reasons for why they need another another pair of shoes, or another scoop of ice cream. It may feel good to do so, but it proves very little.