By conforming to round surfaces they allow you to plumb directly into 5-gallon buckets, brute trash cans, or any round surface. They accept standard Sch. 40 or Sch. 80 PVC and allow DIY projects that once may have been very costly to complete much more affordable.
Uniseals are used to attach pipes to just about any container in situations where bulkheads will not work. The most common use is on curved surfaces such as storage drums, buckets or even another pipe.