It isn't helpful to say they were designed for different purposes - they were meant to serve complementary ones. ABC was designed to express a large and useful subset of what you can by "standard" notation (which in fact is very un-standard, shading into downright paper artwork with things like Trevor Wishart's "Vox"). Exactly how large that subset is depends on the version, the implementation and how far the user is prepared to use weird tricks. (I got Phil Taylor to add quite a few features in Barfly because I wanted them for the old Scottish music I was transcribing).

Conversely, if you write staff notation in such a way that ABC can represent it, you have a better chance of it being portable to other notation systems, not just ABC. (In particular, if you get formally precise about bar lengths, you will be doing the world a favour; the way some traditional notation plays fast and loose with the lengths of upbeats is not a desirable tradition). And some ideas originating in ABC can also be taken over into staff notation: writing mode names explicitly, using the part construct to abbreviate, thinking of ornament patterns as macros.