Why do we round numbers? Because of our inability to process data that's very detailed. We chop off some details that we think are unimportant and we keep the rest. We don't round only numbers; we round everything. Books, for example.
We know that there are billions of books in the world. And all of them are unique. Every one of them is different from everything else. Is that because every book uses unique words? No. Is that because every book uses unique phrases? No. Is that because every book uses unique sentences? No. Is that because every book has unique paragraphs? No. Is that because every book has unique chapters? Maybe not. If you have ever tried writing a book, you'd know that what reaches the readers is only one version what you had written. You would have written several books that share many chapters, and you published the one book that was the best among them all. So, a book is unique only when you see it from cover to cover as a whole.
But how do we judge books? Isn't it common to say "A few chapters in the middle are a little boring, but it's a pretty good book"? Isn't it common to remember only a few parts of a book and forget everything else from it? Isn't it common to skip a chapter in a book because it's way too boring? Are we not rounding a book when we judge it by excluding certain parts from it? Why do we always want to judge an incomplete thing than to simply not have an opinion about it?
Same applies to people. A person is what they are, what they do, what happens to them from their birth to end. I think knowing even oneself is extremely hard without rounding... let alone knowing others. But how we have put everyone we know in buckets like "good person", "crazy dude", "smart chick", etc!