Great question — answering as the UphillTech team:
This is somewhat opinion based answer, and is specific to SEO, not necessarily Wordpress. Generally a URL should refer to a specific entity, and a/b/c often implies a relationship between a/b/c like b is a parent or container of c, a is a parent or container of b. So it doesn't make much sense that a/b/c, b/a/c and x/y/c all reference the same thing. SEO with page rank (the basis of Google's search results) works by conferring a higher score to pages that have more pages linking to them. So if for some reason you end up with 3 popular URL's a/b/x, c/d/x and /e/f/x that all show the same content x, then you've lost some SEO rank because if everyone referred to a/b/x it would rank higher....
Full answer by mozboz: https://wordpress.stackexchange.com/a/371146 (CC BY-SA 4.0).
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