Many workers in developing countries are crushingly poor, malnourished, and have virtually no access to medical care or education. This poverty and deprivation causes great suffering.
Many workers producing some of the staples of the Western diet — coffee, tea, chocolate and bananas (the main fair trade products) — are desperate and have no other options, so they will work all day for just enough wages to survive. As long as the workers do their job and don’t die, the corporations selling these staples profit. Since they can pay them so little, they do, because that makes them a larger profit.
Besides paying them more, fair trade companies and certification agencies monitor the farms for human rights abuses, and invest in their communities in various ways to improve their lives.
The human rights aspect is significant, because in the chocolate industry especially, child slavery is rampant. Farmers kidnap poor rural children, take them across the country, and force them to work on chocolate farms for years for no pay. This horrifying practice is widespread and very well documented (there’s an interesting documentary about it that you can watch online for free). Fair trade ensures that when you buy a bar of chocolate, you are not supporting slavery.
Fair trade is a very small slice of the marketplace, and overall it is probably not a very effective strategy for solving the problems of poor people in developing countries. However, it does improve the lives of the workers directly involved.
I would say it is not required, but it is praiseworthy and an act of chesed to buy fair trade. But it would be even more praiseworthy to work towards more permanent international or country-wide solutions to these problems.