So what functions do those all-in-one chips take on, as opposed to chips like the TL-072 or NE-5532?
Oh boy.
Not an expert, but I'll give this one a stab. I'm going to try and simplify this as much as possible, and I realize that some of the more technically knowlegable amongst us will have my head for this, but here goes my vastly simplified half-assed explanation ...
An opamp can be thought of as the building block of sorts, for things that require amplification, filtering, signal manipulation, etc. etc. etc. Opamps do this through a series of transistors ... with the idea, basically, to amplify stuff. In an "operational" way, of course.
By itself, an opamp can't perform this function very well for amplifying, say, a microphone signal. Basically what winds up happening is it amplifies not only the source, but all of the DC, noise and other common-mode votages. So all you get, basically, is a lot of noise ... along with some of the signal you're trying to amplify.
Now in order to amplify what the mic is trying to pick up, you need to have a way of canceling out all the other junk. This normally requires a series of opamp circuits. And what an instrument amp does is sort of pre-packages all of these circuits in to one monolithic piece ("integrating" the circuits, if you catch my drift). First, it does the whole "gain stage" thing (taking the signal and amplifying it) followed by another stage that removes the "common mode" (basically filtering out all the junk).
To be more precise, it first amplifies the difference between two input circuits, and then rejects anything that is common to them (the junk). An opamp (like a 5532, etc.), by itself, would amplify
everythig -- including the junk. And a lot of people think the 5532 is junk btw, but that's a whole nother topic. Anyway, if you get how a balanced signal works, then it shouldn't be a stretch.
So in a very overly-simplified nutshell, the instrument amp is doing all the grunt work of taking the relatively weak signal coming from a transducer (in this case, a microphone), and amplifying it to a
useable signal. Basically, it is performing the function of a
mic pre. Everything else (in the design/layout) just kind of supports what it is doing. Hopefully in a positive way - and therein lies the differentiating factors. bla bla bla.