Nope you should just only eat potatoes

it sure does, especially if you aren't getting the ones you need through a healthy diet.
For quite some time I hadn't been taking any vitamins and recently started up again, I've noticed a difference in my skin (complexion), in healing and in some mental health capacities (sleeping better, more energy etc.)

Vitamin supplements are best avoided. In the UK at least deficiencies are rare and there is neither need nor therapeutic advantage in taking supplements. Recently there has been increasing concerns about what id effectively overdosing on synthetic vitamins. The fat soluble vitamins are the most potentially toxic, A to the eye, D to the kidney K to the blood clotting mechanism and E appears to increase all cause mortality. The most recent, and one of the most worrying papers concerns women taking multivitamins. There appears to be an associated 19% increase in the risk of breast cancer in women on multivitamin tablets, though further investigation is required to confirm that the risk is causal.

Of course it does. Depending on your diet your body could be starving for certain vitamins that you're just not getting enough of.

For example, a vegetarian who works a night job is very likely to have a Vitamin D deficiency, which can lead to bone decay and general fatigue.

Everyone should take a multivitamin.