Making us feel they spent their ammo. That we have breathing room before they target us again. Hoping that our agencies focus on attacks they are planning in the middle east as opposed to focusing on homeland attacks.
They want the world to know they aren't all gone and they still hate us, and will try to kill us wherever and however they can.
They're into activity, not into subterfuge, so they couldn't care less whether we are alerted or not.
Before Sept. 11, intelligence heard lots of "chatter" about pending terrorist attacks, but they assumed it meant in foreign countries, not in the US.
There is currently a ton of "chatter" (on the level of Sept. 11)... So if they do a few bombings in foreign countries, they are hoping the intelligence community will again assume that the chatter is about foreign locations. The terrorists are using those bombings as a diversion - so we will presume America is safe since the chatter was about foreign targets.
Think of it as a crook calling a bomb threat for three shopping centers... and while all the law enforcement in the world is heading towards those shopping centers, the crook goes and robs a bank on the other side of town.