The title holds for much more specific and thinner stacks, along with support, QA, and sales - especially even simple web apps.

Onboarding is important Training is important Retaining is important

Maybe your system is 100% documented, conventional (looking at you Rails, Angular), debt free, tested, instrumented - but more than likely it's not.

But if you get down to staff who can't teach a a system, including product teams that don't respect feature overload, value, or internal feature training. If you prioritize new features over team development and training (aka a team that doesn't know the system), you're likely to get muddy with existing features both technically and use facing.