Publicity. To get on the news. These guys would never bother to kill random people if nobody ever heard about it. There would be no point.
Terrorize the population: to manipulate governments to act a certain way by targeting innocent civilians. The definition of terrorism.
Recruits: the more violent, cruel and inhuman they make themselves appear, the more they can get anti-social outcasts, especially from Western countries, to join them. Those terrorist recruits with European passports are very valuable.
To polarize the population: they count on the inevitable backlash against immigrants, refugees and Muslims in general to drive some of those people towards extremism. The more Muslims they can get Western society to alienate and reject, the larger their pool of potential recruits. A less tolerant society is a boon for them.
To start new wars: More Western military misadventures is good for terrorist business. The longer Europeans and Americans hang around in the Middle East, the more the terrorist narrative of defending Islamic lands from invasions will seem believable. The propaganda value is great. None of this would be possible without for example, if the Israeli/Palestine conflict had been settled.
To cause instability: terrorism thrives on chaos. In the early 1980s it was Lebanon that was going through a civil war, and consequently Beirut was terrorist central. Next it was Afghanistan, in two waves after both Soviet and US invasions, and now it is Iraq and Syria. Each time a state fails, terrorist groups appear and thrive, filling in the vacuum. If they can get some Western countries to take out yet another stable government in the Middle East, that's another corpse of a country for these maggots to feed on.
The 19-year old third-generation Algerian French kid who goes on a suicide mission is obviously not making a very rational decision. But the masterminds who have recruited and manipulated that kid know exactly what they are doing.
In short, this is what war in the 21st century looks like. As physical borders become increasingly porous, and old notions of national identity begin to dissolve, the traditional notion of warfare also goes obsolete. The new wars are fought asymmetrically, by nebulous widely-spread networks with no physical power centre.