Why?

The two horrific suicide bombings on Sunday in Baghdad may have killed 150 or more people. What I don't get is the motive. What is accomplished by such acts? Can they really be meant simply to demonstrate the Iraqi government's weakness? Surely they can't be meant to attract any kind of popular support - and it seems to me they only serve to slow down the U.S. disengagement from the country. What possible ends do such atrocities serve?

Photo by Joao Silva for New York Times; at http://www.nytimes.com/slideshow/2009/10/25/world/20091025-BAGHDAD_index.html