Local terrorism in Iraq/Syria and the region is very effective, as terrorism feeds extremism. It can destabilize the government and help create the kind of power vacuum that inevitably leads to the most brutal group (presumably the terrorists) being the only ones capable of maintaining discipline and restoring order. In a situation like Iraq or Syria, moderates have the majority behind them but their fractured nature makes them inadequate once a civil war has turned to many-sided brutality -- moderates just aren't good enough at brutality to win at that game, and their soldiers hear the brutal stories and run away from their posts in fear.
Without 9/11 there would probably be no Islamic State, so clearly they can accomplish a lot with remote terrorism as well. It can draw a country into helping destabilize a region where the terrorists can take advantage of the chaos. It's also always useful to make your enemy panic and devote their resources to trying to defend against the indefensible. And if IS can make enough panicked westerners blame and shun Muslims, the shunned outcasts become much easier to recruit.