There are two possible positive interpretations, and the cynical one:
Bombing as retalation, signaling Daesh that if it does not stop terror attacks in Western Europe, Daesh will be bombed weakening them. It assumes that Daesh has a tight control of terror cells, and that it will find that the bombing does more damage than the political/propaganda gains it can get with the terror campaign. IMHO, since the Daesh regime should have expected such retalation, it means that they think the terror attacks are worth the bombing damage (unless Western analists find some unexpected way to hurt Daesh bottom line).
Bombing as a plan to weaken and finally expel Daesh from the territory it holds. Having a "free harbour" where radical Muslims can openly meet and receive training, funds and/or materiel helps in the preparation in terror attacks. Someone training with AK-47 shoting in the outskirts of Paris is bound to attract police attention, which means capture risk and less opportunities for training. Asking people to join you for suicide attacks in Amman is likely to get you denounced to police. Weakening Daesh until it is overrun by Iraq/Kurds/Syria would stop that.
The issue with that is that bombing is not enough, you need troops (yours or allied) in the terrain and to commit to a stabilization process, with allies like either Assad's Syria or the notably unreliable Iraq. Look a Libia for an example of what could go wrong.
Anyway, in case of success you could not guarantee that there would be no more attempts at terror attacks, but you could trust in more of them being discovered before they are performed and the ones not detected being of a smaller scale.
The cynical interpretation is that politicians must show that they have a response, even if they do not have one (or they have one but are unwilling to commit themselves to it because it involves painful or unpopular decisions).
Even if they know they are not really making a dent, sending some bombers to kill someone (hopefully, terrorists and not "collateral damage") in Syria and Iraq will make some people "feel good" and that the country is still powerful, which will mean more votes in the next election that if the government just does nothing.
For the "military-industrial complex" theory, I do not think those operations are lucrative enough to justify a conspiracy theory: some ordenance will be spent, maybe an aircraft or two will get damaged... it will sound like a lot of money if you compare it with your salary, but most likely it will be just peanuts compared with the regular defense spending.