It allows you to work with massive amounts of concurrency. Nobody sane would create one hundred thousand system threads, but you can do this using stackless.
This article tests doing just that, creating one hundred thousand tasklets in both Python and Google Go (a new programming language): http://dalkescientific.com/writings/diary/archive/2009/11/15/100000_tasklets.html
Surprisingly, even if Google Go is compiled to native code, and they tout their co-routines implementation, Python still wins.
Stackless would be good for implementing a map/reduce algorithm, where you can have a very large number of reducers depending on your input data.