It is used for bedding for cattle though some straw is usable as feed. Barley straw fetches between �30 and �40 a tonne. Straw is even imported sometimes from abroad to make up shortages. The . burning was old agricultural practice. It was thought that it avoided any disease developing, there was far too much to plough in with good effect and that , in any case, it was time consuming to send out a baler to stack it all. If it was burned the remaining stubble and ash could be ploughed back in though and there was some residual nutrient in it.
A lot of the stuff you see in plastic bags is simply material, usually grass, which is to be prepared as silage that is it is stored under pressure to make it into digestible winter feed.