Much of what passes for testing is a near-criminal waste of corporate resources. I've seen so much rubbish test code.
On the other hand, when the team has a KPI "we need a big test coverage!" and people are lazy or simply bad developers, you end up with a code like
try {
runTheWholeApp()
} catch (e) {}
assertTrue(itRan)Well actually if the organization is that dysfunctional it's probably game over, but Mutation Testing can catch you when you do this accidentally.
Refactor code, and it gives you that much more confidence you have not broken anything.
They're seriously useful mid-later stages of a project, which is probably why startups devs aren't doing them...