While it wasn't as flashy as the others, long range communication, and the ability to mess with someone's head are not to be underestimated. It was fairly useless on its own, but a powerful force multiplier on the others.
I think he just didn’t exist when he “left”. Their rings don’t summon him from another place, they form him. When the mission is over, he ceases to be. There was an episode where the fire guy goes back in time and prevents himself from getting his ring and creates an alternate timeline where Captain Planet never existed at all, because the other planeteers can’t “cast” him without fire.
There was an episode that showed an alternate timeline where Wheeler refused the fire ring, so the planeteers were never able to summon Captain Planet and couldn't defeat the villains, so they ultimately went their separate ways but kept the rings.
Ma-ti is shown using the heart ring to essentially mug people, altering their minds to force them to give him money.