Magento2 What's the point of having both InstallSchema and UpgradeSchema

I got this question in a training I was giving, and we tried out if we need InstallSchema / InstallData at all. It turned out we don't need it, as the Upgrade classes are executed on installation too. So, in my opinion, I would not use the Install classes at all as the distinction is quite arbitrary and it only confuses in most cases.