Short answer is many-- if not most-- people in the US are probably deficient in some micronutrients / vitamins & minerals.

The body uses different vitamins & minerals to facilitate different body and cellular processes. If you're short on these nutrients, the body just doesn't run as well (like a car wearing out quicker if not well maintained). It may not be apparent for some time, but you'll just age a little quicker, and maybe die a couple to a few years earlier. Unless you're tracking it in an individual, how would you know if someone who dies a little early-- say 50 or 60 years old-- might've lived another 10 or 20 years if they ate better. We sort of intuitively know it for some people (they ate like shite and had a heart attack), but sometimes we don't see the slow damage.

If you eat a perfect diet, which can vary person to person, then you might well live 80 to 100 years with few health issues. In this case, you'd have no need for vitamins & mineral supplements. You'll probably piss a lot of it out if your body has enough.

But this is harder than it used to be. Avoiding processed foods & sugar and trying to eat more vegetables is traditional advice to get more vitamins. But take this into account:

Soil Depletion - Scientific American
or
Veggies have less vitamins - Nat Geo

Modern farming practices deplete the soil and for a lot of store bought produce, you're just not getting the nutrients you thought you might be getting anymore. I haven't read about it, but imagine this holds true for meat as well. If a cow is corn fed vs grass fed, the diet is going to dictate nutrition levels in the meat itself to some degree.

So, vitamin and mineral supplements can help. A multivitamin can be better than nothing. If you want to get granular, some vitamins / minerals ideally should NOT be taken together as they sometimes compete for the same receptors in the body, others are better in the morning vs night, some forms have different effects (D2 v D3, K1 vs K2, Mg Citrate vs Mg Threonate/Glycinate, etc.).

The Health & Fitness Board here on TD often has a lot of discussions about supplements and a lot of people who know more than me about best brands and combos, what to avoid, getting bloodwork done (if you're going all out), and so on:
Fitness Board