Drainage

• The most common reason you might find a drain in your backyard is to -- shockingly -- provide drainage. Some backyards do not have sufficient natural drainage to prevent water from pooling or even flooding the yard, so some homeowners install drains themselves to facilitate effective drainage.
  
  

Remains of Old Construction

• Some drains may just be the result of the destruction or deconstruction of an old building in which there was plumbing. Drains are not especially easy to remove once installed, so it is possible that the house's previous owner just decided to leave them in after tearing down the attached building.
  
  

Part of Outdoor Plumbing

• If there is a water standpipe nearby -- or a piece of plumbing like an outdoor shower -- then the drain may be there to provide drainage for that piece of plumbing. This is slightly different from Section 1, because it is designed to drain man-generated water rather than the natural run-off that occurs during rainstorms or the like.
  
  

Poor Design

• It may be that the existence of the drain is just a massive miscalculation on the part of the previous owner, and that it serves no purpose whatsoever. Perhaps it was designed to provide drainage in a place that needs no extra drainage, or perhaps it was put in in the anticipation of new construction.