T-shirts, posters, and other non-media merch has a better profit margin for bands. Shirts normally cost 3-6 bucks to make and they can easily sell them for 10-15 dollars. CDs and records have normally only a 1-2 dollar mark up. When you're on tour, taking shirts are a lighter load when space is tight. I'm not saying don't bring/sell cds, but shirts will normally outsell CDs when you're touring.