From the point of view of the western politics: the threat of terrorism is often used in elections and to justify political actions - understanding the threat of terrorism helps understand the political situation of a country.
From the point of view of the terrorist: they use it as a weapon and a way to attract more members.
From the point of view of the people: terrorist attacks such as in Madrid or NY are a threat to lives, and a threat that needs to be controlled and acted against.