Augmented triads use, effectively, a major third stacked on another major third. Thus there are really only 4 of them. E,g, C-E-G# is the make-up of C+, but also an inversion of E+ (using E-G#-B# ), and G#+, (with G#-B#-D## ). The names of the notes have to be changed, technically, but the sound is the same.There is another 'starting' on C#, then two more, on D and Eb. After that the cycle starts again.
Thus they can be interchangeable between keys, rather like diminished chords, which effectively use a minor third stacked on another minor third, making 3 of them before they cycle round to repeat themselves in inversions of the same notes.
I know the augmented actually uses a maj. 3 and an aug. 5., but I'm trying to portray the mix in a different way.
Having said all of that, the usual modern use seems to be as a sort of dominant, moving, for example, from C maj-through C+ to-F.The sound is too unstable to stand on its own.
So, using an aug. chord, the tune can stray into another key - or modulate.