Goats like variety some do like eating straw- good roughage along with the hay, I put some straw in some racks with hay in the rest of the racks often the straw comes out of the racks more easily than hay -they have a self bedding system! I think the extra roughage just like tree branches slows the process through the digestive system so they get more nutrients out of the hay, spring greenery, concentrates they are eating. Mine love eating straw as well as their ad lib hay. Some people might say the hay you have may be poor quality so are forced to eat straw, which may be true in some cases but if your hay is good quality and your goats eat it then they just like the variety also they are individual in their likes & dislikes.