Will it make my Shure 530's (which are in their database as I saw already) sound "better" and as they are supposed to sound ?
As they are supposed to sound, meaning "flatter." They just take the guesswork or repeated measurements to make a headphone "flatter."
Let's use for example how we tune our cars. Whatever system it is, let's assume for a moment that you have the standard digital processors, which after setting crossovers and time alignment to take out as much of the car's acoustic environment as possible, leaves you with the EQ. So you hook up a USB microphone to a laptop, tape that to your headrest, then you get a baseline. You see where there are peaks or wider plateaus, or dips and wider "valleys" in the response, so you apply the EQ. That isn't straightforward given the EQ bands you can control aren't exactly where those aberrations in the response are, and even if you have bands within a wider range, widening its response means you might end up not getting through the entire plateau or valley....or you go beyond it. So you repeat the process, measuring and tweaking and measure, and also using your own ear to determine if you might actually made it sound worse or too sterile.
Now imagine trying to do that at home. Most people who do that in cars tend to really blow their money on it for two reasons: either they compete in competitions, or they spend enough time in their cars on workdays and just want to sleep by the time they get home (it's not surprising this is big in SEAsia, I blame traffic for it), in which case they're alright with spending $100++ on a USB mic or can borrow from someone they know. Most people with headphones might not see the point in spending $100 on a USB mic, let alone a ballistic dummy head or some other way to isolate the sound of having earpads isolating the sound output as they rest on your cheek (or in the case of IEMs, how the tips rest in your ears).
Accudio, if you trust that they did it right anyway, saves you all of that work. You can just tweak it to your own tastes, also because how we wear our headphones and IEMs are a little differently from one person to another.
Or is the main purpose of the app to make cheap(er) earphones to sound like more expensive ones ? If the latter is true, I assume that buying the app is not worth it for me.
It's just a byproduct of the program. If you know the baseline of each to make them as flat as possible, then you can also do a simulation to make one baseline sound like another baseline. Now, just because it can simulate frequency response, does not mean you can simulate all the sound characteristics of two headphones. For example, I can set my HD600 to HiFi simulation (just right) to flatten the response, or even to simulate the HD800 sound (a bit too flat). This does
not mean they'll sound the same - even in frequency response simulation, the 600 sounds too flat compared to the 800. Next, there's the matter of the chassis effects on imaging and soundstage - the HD800's earcups put the drivers farther out from the ears, and angled like speakers. No EQ can simulate how much wider and deeper the soundstage is (not a lot), or how much more forward and out of your head it is (not by much either).
Not even just the EQ tweaking the frequency response can make the earbuds that came free with your iPhone sound like an SR009 - the mere fact that the driver on the cheap earbud
may be distorting (not necessarily doing the "thwack-thwack" overexcursion on bass, but the diaphragm won't be as smooth while pushing in adn out) as it moves, or for a dynamic driver cannot move quickly enough, cannot sound exactly the same as a vibrating electrostat.
