I can only speak about Delta, however most of the major airlines have very similar rules.
Why you want 1st class domestic:
You do get lounge access, doesn't matter what level you are or even if you are a member of their frequent flyer program, first class gets access.
Short flights less than 3 hours anything from cookies/chips to a light snack. Longer flights hot meal. Of course all free.
All drinks free.
Extra baggage, a biggie for some of us!
Free pre-boarding. Remember some are now charging to let you on early.
You don't have to pay extra so sit next to your loved one. Many airlines now charge extra for isle/window seats in the back.
Yes kids are allowed, but rare to see any. To be honest another reason I like the front.
As for United, I would take a bus before I would ever use them again. However that's just a personal thing with me, some people have had better luck than I with them.