What's the point of flight insurance?

Please if you work for airlines or have any intell, I would really appreciate it. Every agent is giving different information and I'm going bananas.

I have an upcoming flight soon with my bub, son and I. Now because of unfortunate circumstances my son will be heading to San Francisco a couple of days before for a funeral. I want to have him fly from San Francisco the same day as my hun and I fly from home.

I do all my bookings through Expedia and they referred me to the airlines.

Now here's the pickle: I told them I want to cancel his outbound flight but keep his returning flight. One agent says to just show up, tell them he's not flying with but to save his return flight. That already makes me uneasy, I've dealt with issues at the airport and they usually want to charge fees, or have no more flights available and are forced to stay an additional day or more.

I called again and they want to charge me $200 for the modification, mind you a one way flight from Oakland to Vegas is $20 dollars.

I remind them I paid insurance but they're giving me a bunch of excuses.

I don't care to pay extra but I'm wary I won't find him a flight that returning day if for some reason they mark him a "no show" because of his outbound flight.

I'm just wondering what to say to them that'll ensure me,we will all fly back together. I'm just at my wits end 😔.