A short answer is the last paragraph.
Born and raised as a Muslim, though I am more inclined to defend my lifestyle choices with progressist politics, rather than entities with extra-political world views, like ISIS, I shall attempt to answer your question.
First, ISIS doesn't do all the attacks (it just claims to do so) and not mainly for inflicting fear on its targets or anything because, as your question presumes well, against NATO-like military organisations, no movement - no matter how well equipped it is - can resist and though it is unreasonable, ISIS knows this for a fact and thus it tries to become a symbol. ISIS promotes two different images to different societies.
In my opinion, what ISIS does is to try to appear as a "hope zone" to those who are marginalised by the very society in which they live. They openly target the second generation of immigrant families and prisoners for recruitment into wealthy societies. The first generation of immigrants, those that arrive, most of the time are just grateful to be alive, maybe not in the conditions, they had hoped for but also in no position to be attacking the very fabric of the society in which they have been received. I am a first generation immigrant, and that is how I see the people around me, I can clearly weigh my options and see that my prospects of living here are more reasonable than my home country.
The second generation, though, for the most part, has a severe identity crisis. This identity crisis for some cases results in major isolation from the society. That's where a search for something starts. For some that something results in Isis. A poor choice in all prospects, but some go along with it. The possibility of being part of something, like anything, especially something that is condemned by the society from which you feel isolated, must be thrilling for some people. Even the thought probably gives excitement to some. One also has to admit that ISIS acknowledges this, that is you do your part, go explode somewhere, kill a lot of people, then ISIS does its part and acknowledges your "achievements" by declaring you as part of them, and their cause.
It plays a totally different game in near-east countries though. It also attacks those countries too, by the way. Turkey alone had seen 23 ISIS related bombings in 2014-2016, or Iraq had seen 15 of them in the first 6 months of 2015 or 2014, I don't remember the year really well, but media in here doesn't talk about it if it is not European among the dead. In near-east, it uses the misery of the life conditions of the people. Think of it this way: you are in your early 20s, you have no money, you barely survive by working in long shifts in poor conditions, with no money or a decent job, you have a very poor chance at marriage if you are an average looking guy or woman, and let's say you have been raised with conservative values, though not being necessarily very well versed with the interiors of the religion that is supposed to be the basis of those values. In all that misery, some colleague says one day: "hey I have this friend who is organising an event for discussing such theme in some locale, would you like to come?" As time passes on, this friend proposes you the following option, become a soldier of ISIS, have a wife from their captured sexual slaves, have a steady income, live in the designated houses either for very cheap prices or free. You are promised to die as a martyr of Islam, which is a very high status for some.
To answer your title question directly, ISIS doesn't really care about presenting itself as the enemy, as long as it can appear as a community, to those who are marginalised by the targeted countries, which destabilises the society of the targeted countries. And let's face it as long as the Syrian civil war exist, they will continue to exist, and probably even after that, since the civil wars in Libya and Sudan also are going on in the region, they have high chances of continuing their organisation there as they feed on the misery of the people for their staff.
As for the question of why reproduce the Al-Qaeda methods, well... They are not reproducing it. Al-Qaeda was strictly against non-Muslims, so countries like Iraq, Iran, Egypt, Turkey, didn't really suffer from that organisation. According to Al-Qaeda, the enemies were and still are the non-Muslim countries. For ISIS, the definition of the infidel includes different sections of Islam, you might find it hard to believe, but in Turkey, the most ardent attackers against ISIS came from extreme right and radical Islam, since they refused to be branded as infidels, just because some new kid decided for them to be so. Plus accepting ISIS would mean a loss of control in their communities, etc.
Regarding the use of terrorism by both organisations, my personal belief is that both of the organisations are a disgrace to Islam that I have been raised with, but their attacks don't necessarily appear as such to their relative communities. Think of it this way, bombing ISIS for saving Yezidis was an incredibly human gesture of US that would be remembered for a long time among Kurdish community, but the bombing of Iraq would not be considered so. The action is the same, but its perception differs among the people. The same goes for the community that might be willing to associate themselves with that movement, those actions are not perceived as terrorism. They differ considerably by the way. Al-Qaeda is still against the attacks in regions like Pakistan, Egypt, etc. For Al-Qaeda, the attacks in those regions are what you would call terrorism. Though this doesn't change much as far as the non-Muslim societies, I am just trying to say that they do differ in their perception of the enemy, but not in the choice of action. Why choose terrorism at all you might say, well that is quite easy. It is the only course of conceivable hostile action that would put you into the headlines if you are an organisation from the Middle East.
