Why?

According to the Encyclopedia Britannica, terrorism is:
'the systematic use of violence to create a general climate of fear in a population and thereby to bring about a particular political objective'.

It seems to me that they are mostly right. For instance, the original objective of terrorist movements such as the IRA or the ETA was to gain the right to rule their country. In case of ecoterrorism, the objective is to stop activities that may harm nature, such as the hindering of nuclear transports.

However, the above definition appears to be woefully inadequate when it comes to the terrorist activities that we have seen in the 20th century. At no point in time have I ever read any that gave me an idea as to what the leaders see as the objective of their acts.

Take today: 14 people have been arrested in Belgium on the suspicion of terrorism. They are supposed to have connections to Al-Qaeda. Apparently, their target may have been the Euro-top which is taking place today. The question then is why? Is this on religious grounds? Monetary? Repression? Politics? And what do they hope to achieve?

I really don't know, and I really don't understand. If you have any clues and can fill me in, please leave a comment.