The problem with obtaining the mineral selenium is that whilst many foods contain it, the amount varies enormously. There are areas of the world where soils are very low in this mineral and the same goes for the sea – thus even though seafood and land-food can contain good amounts, they only will if it is present. The reason we tend to have low levels is due to the modern diet coupled with the elusive nature of selenium.
Selenium is needed for correct cell function – therefore it is vital. As with all nutrients, they work with others as a co-factor in order to make anti-oxidants. Without these we cannot fight disease and become more prone to infections, cancers, heart disease, cognitive decline and more. See this article.
The thing to do is eat the correct diet for humans (see my blogs), buy organic or grow your own. For the rest, buy food from various retailers/producers thereby getting produce from a variety of geographical areas.
As usual, I recommend obtaining nutrients from organic food in preference to supplements, but there maybe a time when it is necessary. See a specialist!