I’d highly recommend getting good workout clothes for a few reasons.
If you’re working out, you want to get healthier or look better, and liking your clothes will make you more confident and ready to work hard. When you don’t feel like getting off the couch, it can help to have some clothes you love.
It can also be important for health reasons. Cheap cotton t-shirts (especially the free ones from school that everyone runs in) soak up your sweat and hold it against your skin. This can actually clog your pores or cause infections. Athletic material clothes (like polyester and Lycra) are meant to pull that moisture off of your skin and allow your skin to breathe.
You’ll look better and feel better if you have the proper clothing, so if you’re serious about getting healthy, get some serious gear.