Engineering is about building and maintaining things fit for their purpose.

A bridge that only needs to last for one year is designed very differently than a bridge intended to be used for twenty years.

In the case of software, that means (a) building what your users and business need; (b) designing software that is easy to change; and (c) delivering enough scale to meet your expected needs for the current business cycle.

For a new startup, you can probably get by with a Rails app on a single Digital Ocean droplet, with plans to scale horizontally. For an established business, you might have a large data warehousing operation running custom C++ code across multiple datacenters, of which the web frontend is a tiny (but important) part.

Good software engineering makes the steps between "New Startup" and "Established Business" as painless as possible, with a minimum of additional overhead.