We build tarps because they are one of the most useful pieces of outdoor gear one can bring along.
Whether it's rain or sun you are trying to get some relief from, a tarp can make a big difference in comfort and fun. In an all-day rainstorm, a group of people can hang out, tell stories, read books, cook and eat, and just plain enjoy themselves. The alternative - a camp without a tarp - is separate pods of tentmates hunkered down inside tents wishing and hoping for the rain to stop so they can come out and party.
Paddling and rafting trips are well known for the inclusion of tarps, but backpacking outings should also include this great item of gear. With a few trekking poles a very nice group shelter is just a few minutes away.