and a good analysis of "why and what" here
http://dailykos.com/story/2004/11/10/01247/557excerpting a bit:
In radicalizing your apathetic sympathizers, you have no better ally than the violent extremists on the other side . Only they can convince your people that compromise is impossible. Only they can raise your countrymen's level of fear and despair to the point that large numbers are willing to take up arms and follow your lead. A few blown up apartment buildings and dead schoolchildren will get you more recruits than the best revolutionary tracts ever written.
Perversely, this means that you are the best ally of the extremists on the other side. That doesn't mean you love or even talk to each other -- they are, after all, vile and despicable demons. But at this stage in the process your interests align. Both of you want to invert the bell curve, to flatten out that big hump in the middle and drive people to the edges. That's why extremists come in pairs: Caesar and Pompey, the Nazis and the Communists, Sharon and Arafat, Bush and Bin Laden. Each side needs a demonic opposite in order to galvanize its supporters.
Naive observers frequently decry the apparent counter-productivity of extremist attacks. Don't the leaders of Hamas understand that every suicide bombing makes the Israelis that much more determined not to give the Palestinians a state? Don't they realize that the Israeli government will strike back even harder, and inflict even more suffering on the Palestinian people? Of course they do; they're not idiots. The Israeli response is exactly what they're counting on. More airstrikes, more repression, more poverty -- fewer opportunities for normal life to get in the way of the Great Struggle.