Picking the right drain valve can be tricky.  You need to consider initial cost, product reliability, operating cost, and any constraints that are unique to the application.
 
Auto drains remove condensed liquid – usually a mixture of water and compressor lubricant from compressed air systems.  Automatic drains are commonly installed on air receiver tanks, drip legs, bulk moisture separators, coalescing filters, refrigerated air dryers, and deliquescent air dryers.
 
As the name implies, auto drains remove human labor from the mix.  Buy an automatic drain valve and a maintenance worker no longer needs to patrol the plant, manually dumping liquid from the air system.