The perfect diet would give everyone all the nutrients they need without supplementation, but who has a perfect diet? Is it even possible? Here are the reasons the right kind of supplementation is always beneficial in reaching optimal nutrition:

1. Even if we knew what the perfect diet was, nobody would be able to be perfect and achieve it. Eating all the right foods in the perfect combination, avoiding dining out, special treats, holiday and social occasions, avoiding alcohol, sugar, fat, salt, etc.
2. Foods grown in deficient soils, or not grown to the peak of ripeness may not have all of the nutrition to start that is supposed to be in the food.
3. Nutrients are lost in food transportation, storage, cooking.
4. There is individual variability of nutrient absorption.

It's important to note that the "right supplementation" is from "whole foods" (for example: fruits and vegetables) rather than individual nutrient extractions from food (example: vitamin, mineral, or phytonutrient), or laboratory-created nutrients. Research shows that healthy whole foods work to benefit health, when the nutrients in the foods given separately from whole foods in "isolated form" in many cases do not achieve the same benefits (and can actually be harmful such as with vitamin A or E). This is because of the synergistic effect of all food nutrients in a balanced "food complex" which work together to give the positive health benefits. These include vitamins, minerals, phytonutrients, and unknown factors (in the proper ratios). Adding individual nutrients can in some cases "throw off the balance" of nutrients in the body and cause negative overall effects.

Thus, there is merit in the right type of supplements that have "whole food concentrates" containing the substances that research shows can be beneficial to human health. These can be used to "bridge the gap" between our best efforts at eating better--and reaching optimal nutrition.

We know that our soils are depleted. Our food may "look" pretty, but the majority of the time it is lacking the nutrient value that we once had. Even our organic produce is still lacking the nutrient value even though it may be cleaner as far as chemicals are concerned. The once alive and active micro organism layer that used to live inside of our soil is gone. It has been killed off due to over use of pesticides of all types. This layer of organisms was responsible for breaking down the minerals in the rocks and soils so the nutrients could be absorbed in to the plant as it grew. But, now, that whole process has been wiped out because these micro organisms have been killed. (watch the documentary DIRT, if you want to see for yourself).
Not only is there a reduction in nutrients, but we also have a reduction in enzymes, Foods grown are less enzyme active, just as highly processed, and chemically laden foods are missing enzymes. This has a huge impact on our gut health and microbiome, which then has a large effect on how our brains and organs function.
Clinically, watching the difference in what happens when someone eats healthy whole foods, and when some one uses a supplement, the results may seem the same. The key questions are, what whole food is nutrient dense enough, and what supplement is high quality and bio available enough to promote health? It is important to look at the studies, follow the clinical reviews, and most of all, follow how something makes you feel. Are you energetic, digesting well, and glowing in your skin? Or are you fatigued, bloated, and looking sallow? I find that the majority of people want simple, convenient, and fast. Sometimes, the only way to get those things is to use a supplement or meal replacement of some kind. As a Certified Nutritional and Wellness Consultant, my job is to find what fits their life the best, and direct them that way. I do find, over and over, that raising someone's alkalinity a little bit, can reduce systematic inflammation, thus, reducing challenges in the body, I also see many internal issues dissipate quickly.
Personally, my choice is supplementation and also eating clean. And to be honest, the supplementation, always seems easier for my lifestyle, and the majority of my clients feel the same.

I agree with those who have already responded to the query but would like to add a note. Everyone is metabolically unique. Recommended daily intake levels are set for the average population and at a level that will prevent deficiency in 50% of the population. Accordingly, half of the population will likely have some deficiency of some nutrient. In addition, there are genetic differences in how our bodies use and/or store some nutrients. In such cases, supplementation is essential for good health.

The big challenge we face globally is that those who would most benefit from supplementation are those who lack sufficient daily access to food.

To clarify the comment made by Evelyn above, The estimated average requirement (EAR) is the intake that will meet the needs of half of a given population. At this intake, half of the population will have an inadequate intake. The recommended dietary allowance (RDA) is the intake that will meet the needs of most (97%) of a given population. Therefore, the RDAs meet 97% of the populations needs, not 50%. Most commonly, the RDAs are quoted and used.