Why would a certificate authority have multiple root certificates?

When I look at the trusted certificates in my browser, I see that many certificate authorities have multiple root certificates associated with them. For example, org-Amazon has Amazon Root CA 1, Amazon Root CA 2, Amazon Root CA 3, and Amazon Root CA 4.

How do the purposes of these four certificates differ?

I also see that some CAs have root certificates of different "tiers", such as gold, silver, and platinum. What’s the deal with these? I imagine there are price differences associated with the different tiers, but what extra service could they be offering?