What is the reason for hay tarps? It's a reasonable question.
In regions that receive rainfall consistently through the majority of the year, investing in a barn that is large enough to store all of your feed is a very high priority. But even in these areas, there are reasons why you may need to store hay tarps outdoors. Perhaps you haven't built a barn yet. Or perhaps you purchased more hay than your barn has space for.
But in arid regions, where rainfall is infrequent at best, investing in a permanent structure for storing hay may not be a priority at all. Covering with a tarp is all the protection from the elements that your hay may need.
There are two things that you need to protect hay from: spoiling and germination. Allowing grains to get wet can lead to mold, mildew, rot. Any tarp can protect against that. And a sufficiently heavy duty tarp will last long enough to be used and reused for this purpose. But germination requires two things: moisture and sunlight. A good hay tarp will not only protect from rainfall, but it will also be opaque enough to block sunlight from reaching the hay beneath.