Some people do very well in one role but poorly in the others. It's fine to specialize. Some people can switch around. That's fine too. But in any long-term, high-risk conflict nobody can do all the things all the time. Surviving, let alone succeeding, requires a team effort. All the roles are essential to success; you can't skip them. That makes them all important. If you're fighting, you matter. If you're taking time to recover from getting slammed into a wall, you still matter, because recovering is essential to surviving. Have trouble asking for help? Playing this kind of game lets you practice that skill.
It's also crucial to think about context. Some characters are highly resistant to certain forms of damage but vulnerable to others. This is true in some games more than others, and definitely carries over to everyday life. I'm not very imposing physically, but I can use verbal combat to utterly devastating effect: that's one of my DPS modes. I'm not popular, but I am social Teflon; I don't give a flying fuck what people think about me: that's one of my Tank modes. I'm not actually into health care, but that has taught me a lot about how to handle both physical and mental issues on my own, which I can share with other folks, and I do know how to create cozy spaces: those are among my Healer modes. When I feel like crap, and realize I'm just snapping at people whenever I comment, I go find some science feeds to stare at instead: that's one of my recovery modes.
So teamwork involves not just being able to fill a role, but being able to move fluently so you cover each other's weak spots. Say you have two Tanks, one resistant to fire but vulnerable to ice, and the other vice versa. They need to be able to switch quickly in response to different attacks, and the Healer needs to know to focus on the one who just stepped out of combat, not the one currently taking damage. Similarly, DPS need to know their own strengths -- and the weaknesses of various opponents -- in order to maximize the damage they can do. In everyday life, it means one person may have a high-paying job while another has handiwork skills; together they make a more effective household than either alone. A black man and a white woman can have each other's back if he speaks out against sexism and she speaks out against racism. And so on.
Fight the good fight, my peeps. It's a long way to the big boss.