The confusion is probably a lot of us work at smaller companies that serve a wide solution to a niche customer, and that kind of app has a lot of reads and writes but doesn't need to scale. This app might be doing the invoicing/shipments/specialist parts of a business for example.

Whereas there is another different kind of Engineering which I probably will never be a part of (simply due to mathematics of available positions doing it) where you are scaling something up for millions of users but the app is much simpler like a Twitter or Reddit, and the challenge is in the scaling.