Question

Why would you drain an autorelease pool instead of releasing it?

Answer 1

One reason is for garbage collection (not available on iPhone)

release is a no-op with GC enabled, but drain provides a hint to the garbage collector

From the docs:

Garbage Collection

In a garbage-collected environment, there is no need for autorelease pools. You may, however, write a framework that is designed to work in both a garbage-collected and reference-counted environment. In this case, you can use autorelease pools to hint to the collector that collection may be appropriate. In a garbage-collected environment, sending a drain message to a pool triggers garbage collection if necessary; release, however, is a no-op. In a reference-counted environment, drain has the same effect as release. Typically, therefore, you should use drain instead of release.