Generally starting around 25 you're encouraged to get annual checkups. They check for cervical cancer and do a quick breast exam and then send you on your way. It's also a time to go over any irregularities of the past year and to discuss birth control and the like.

Some people also see gynecologists as their regular doctor, going to them for general health checkups.

The US used to have such prevalent hpv rates that annual Pap smears for everyone with a cervix was a better call than testing for hpv. Even today you have to specifically request an hpv test when getting an sti test. The hpv vaccine seems to be changing that, but speaking as someone who had a breakthrough case (fortunately it was a strain known to be low risk for cancer) I still encourage people to not rely solely on it, but also to definitely get the vaccine.

The other reason is that for a long time the gyno was a lot of women’s only regular healthcare visit and it’s still not rare for them to be the most trusted physician in a woman’s life.

My health insurance gives me a discount if I perform a set of preventative-maintenance tasks. They want me to get a pap smear every 3 years, so I guess that's the standard "how often to see a gyno if you have no health concerns regarding gyno issues" schedule.

Edit: I also have a regular doctor I'm meant to see every year.