The assumption of its being religious is simple: it isn't shelter, and it has imagery resembling later depictions of religious figures in the region. Religion here is a more general term to encompass the intersection of culture and spirituality. Was it originally non-religious? If so, why aggregate for months to build a structure, before trade was valuable or very necessary?
Trade becomes more worthwhile when communities begin to specialise. When people are nomadic, they cross many lands, and can simply find what they want on the way (obsidian from the mountains, shells by the sea, berries, roots, and game in forests). When people begin to stay in one place, long distance travel for trade becomes more valuable. As soon as agriculture develops, we discover that obsidian shards from deposits in the Caucasus are found thousands of miles away in central Europe. You begin to have towns focusing on a specific resource, or serving a trade route as goods and people criss-cross the land (Jericho, among the earliest known cities, for example, was possibly involved in the trade of salt; few things can grow in the soil near it).
It is the same as between a spat and a war; gathering in a forest and farming. It is in scale and in character, a different beast.
It should also be noted that Gobekli Tepe was built several millenia before the beginning of the near-eastern Bronze Age. The reason it IS so fascinating is its having been built millenia before agriculture. The systems of religion, administration, and agriculture, did co-develop, but could only do so after the agricultural revolution.