I don't really understand the analogy. Mechanical watches have created an industry of skilled practitioners trained to fix them because they often need diagnosis and repair. NASA has a room full of oncall personnel available for many hours any time they launch anything, and they launch things much less frequently than your average tech company.
Oncall rotations are part of defense-in-depth against bugs and unforeseen circumstances: Most of the companies that survive without a formal one only do so by outsourcing this for the most common cases; to Cloudflare, to Amazon, etc. -- if there's an opportunity cost to being down someone needs to be able to pick up the phone when there's an outage or critical issue.
Oncall rotations are part of defense-in-depth against bugs and unforeseen circumstances: Most of the companies that survive without a formal one only do so by outsourcing this for the most common cases; to Cloudflare, to Amazon, etc. -- if there's an opportunity cost to being down someone needs to be able to pick up the phone when there's an outage or critical issue.