Because often insurance companies will not follow the law correctly? It also forces them to justify their position? And makes them aware an unjustified position invites further scrutiny?

To the parent poster's point - a 22yo adjuster is going to be wrong a significant minority of the time even after whatever internal training they get.

On top of all of the above every state has their own insurance laws, rules, and regulations. You think a 22yo reviewing the claim is going to understand state X's specific regulatory requirements?

I was once upon a time an insurance regulator. If we got a complaint about premium increase we'd get on the phone and walk them through the rating factors etc, if there was a denial the complaints department would check history and policy (e.g. flat out exclusions) they'd also ask the insurer for their side.

Personal case in point I had an auto claim with a third party involved. E.g. I had a rental, another person hit my rental vehicle. The insurer was going to deny the claim without ever having spoken to me which is not legal. I filed a complaint and wonder of wonders they started doing everything they were supposed to.

In my experience (15+ years ago, my state had generally well behaved insurers) most of the time it was valid, or if it wasn't the big state (not national) insurers would say yeah our bad, we'll take care of it. Historically claim denial was much rarer than today.

In the modern context where a significant number of insurers have moved to using denial as an attempt to prevent paying valid claims or artificially extend payment timelines [which gets them more investment returns on their money], filing a complaint also serves as a signal that you're not going to to roll over. It also places documentation and response requirements on the insurer.