What is the point of these large scale terrorist attacks in Europe, aside from what they share in common with all acts of terror?
The first step to answering that question is to read this piece from March by Graeme Wood entitled, "What ISIS Really Wants." It's a very long, thorough, and non-sensational account of their theology. Given that they are motivated entirely by their theology, that's important to understand. The too long, didn't read version is that they want to establish a caliphate and initiate the apocalypse. Their view of Islam holds that all Muslims are obligated to join their caliphate because, and stop me if any of this sounds familiar in re: religious extremists, they alone correctly understand Islam and are, in short, the only True Muslims.
That's a very well researched take. Assuming that the people running ISIS are not naive and lacking any secular political awareness, here's what I think they want.
Europe has a lot of Muslims. Like the U.S. or any other country, European nations have some issues with immigration and suffice it to say that they haven't been welcomed with open arms. That said, Muslim immigrants to Europe appear to find living in Europe on the whole alright. Not terrible. I'm sure they feel (with justification) discriminated against or unwelcome at times but Muslims in Germany, France, the UK, etc. hardly look like they are eager to go back to their original countries of residence. The situation could be better but life in France is far superior to, for example, life in Iran. So the dominant attitude among European Muslims, especially younger ones, appears to be "Sure, this is alright."
Radical extremist Muslims do not like this. They want Muslims to loathe Western society and to be primed for radicalization. They want that for selfish reasons – to grow and legitimize their terrorist organizations. They can neither grow their movement nor succeed in their goals if Muslim kids in Europe are wearing miniskirts and going to music festivals. The problem they face is that while Muslims in Europe face discrimination, they're not treated badly enough to make them hate Western society and governments. They certainly don't hate them enough to want to start killing them. In order for Europe's Muslims to be radicalized en masse as opposed to only a few here and there becoming attracted to "The Cause," European governments would have to treat them worse. Far worse.
Logically, then, what ISIS wants is to push European states far enough to produce a massive anti-immigrant and anti-Muslim backlash. Not just a backlash in terms of attitudes and prejudices but of laws. If some far-right government came to power in France and decided, for example, to round up every Muslim into camps or to force Muslims to carry electronic devices to track their movements then ISIS and their ilk can claim to be prophetic; "See? See? Look at how they treat us!" Recruiting young Muslims to anti-Western and anti-anything other than hardcore Islam terrorism would become a whole lot easier.
That's what I think their real goal is, theology notwithstanding. The worse Western societies treat Muslim immigrants the easier it will be to craft recruitment propaganda. The more indiscriminate large scale killing ISIS does, the worse Western governments will treat Muslim immigrants. Far right, nationalist, anti-immigrant politics are already disturbingly popular in some parts of Europe and they figure that with a little more motivation in the form of random terror with big body counts they can be pushed over the edge into full-throated discrimination as a matter of national policy. In France they see a target nation that has already had race riots and other issues stemming from the social privations of its immigrant underclass. If they rioted before they could be inspired to riot again. The most obvious way to make that happen is to encourage French society and laws to start treating them even worse.
The sad thing is that it might work. I'm not sure they'll succeed in radicalizing many people, but they certainly are making headway toward fueling far-right politics in Europe.