The ILS and the backcourse are two separate elements.
You use the approach button if you'd like the AP to hold the approach for you, but only on an ILS approach, where the localiser and GS station are at the beginning of the RWY threshold. Here, you just need to dial in the frequency information, runway course, and within 10nm of the runway, press APP. This does not land the aircraft for you, it only holds the approach stable to 200ft.
You'd "mess" with the backcourse button, as Tailhook so eloquently puts it, because in some circumstances, wind doesn't always play the part we want it to.
For example, let us suppose that runway 09 at Funky Airport has a full ILS installation. However, the wind for today is 12/260 gusting to 19. That means, that to land on runway 09 at Funky Airport would be suicidal because you'd effectively be landing with a 19kt tailwind. So, we opt to land on runway 27. This end of the runway doesn't have an ILS system, but we are in IMC conditions so really need some sort of aid. Therefore, we can track the runway by reverse sensing (right is left and left is right) the localiser in RWY 9; the opposite end. Here, the backcourse button serves the purpose of the APP button, it'll hold
the backcourse approach, i.e. the approach opposite the runway. Bear in mind, that only in very rare circumstances does a backcourse approach come complete with a GS.
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