You can jump on your bike in any shirt you like. You can bring your old raincoat. It's even okay if you wear Star Wars socks in your bike shoes. But if you ride extended tours wearing the wrong trousers, you are bound to get into trouble. Jeans, corduroy trousers and the like are simply not made for cycling, the fabrics and especially the inside seams will quickly prove uncomfortable, whereas cycling trousers ensure comfortable pedalling. Read more if you want to know why.