The key is in the code that works with objects/classes that implement this interface (this is true for all marker interfaces, by the way). You'll find these if you search through the source files of the Java class libraries looking for "instanceof RandomAccess" (which is how you check an object for whether it implements this interface).
I'll spare you the suspense and point out that it's used mostly in the java.util.Collections class. Have look at its source code; wherever you see "instanceof RandomAccess", it'll invariably be part of an "if" statement that decides which of two algorithms to use. And the one being used if the instanceof check is successful is generally (much) faster than the other one. [ July 24, 2008: Message edited by: Ulf Dittmer ]