Seed potatoes are ones specially grown for planting. They are certified virus-free, which reduces any risks they may carry, which supermarket potatoes could potentially have.
It is always better to grow from seed potatoes. You can buy seed potatoes from late winter onwards. As supermarket potatoes are not certified as virus-free, planting them could lead to your soil becoming contaminated.
Some of the viruses potatoes are susceptible to, such as potato blight, can contaminate your soil for many years. If you do choose to plant potatoes you have purchased from the supermarket despite the risk, try planting them in isolated containers to protect your soil from long-lasting damage.
Seed potatoes of early varieties should be started to grow indoors in February so that they can be planted outside in March/April. In order to ensure you get the seed potato varieties that you want, buy them early.