A typical fruit or vegetable can have hundreds of phytonutrients and other chemical compounds that are just very hard to recreate in processed foods. This is in addition to the 30 or so micronutrients that we already know of that are essential for certain biological processes. In the black hole that is nutritional research, population and epidemiological studies simply show people who do include a lot of vegetables in their diet tend to have better health outcomes. It's likely you might draw a different conclusion from that research than people involved in the field, but it's probably one of the safest bets we can make when it comes to staying healthy.