A/B/Y boxes are focussed more on the output, with the option to send your output signal ( after all effects processing) to amp/path A, path B, or both.
In contrast, a looper assumes there will be only ONE input and ONE output (to the amp or some stage/device that will apply in all instances), BUT there may be either one set of effects inserted into that path OR a different set.
So, for example, you may have a compressor and EQ box set for a bright. clean, and nondistracting rhythm tone in one loop, and a wah/distortion/chorus combo set for solos in the other loop. Stepping on the loop-select switch lets you leave everything where it needs to be within each loop and, in essence, turn all the pedals in one loop OFF and the pedals in the other loop ON with one footpress.
If you use closed-circuit send/return jacks that are "normalized" (i.e., the send jack is automatically connected to the return jack when nothing is plugged in) then the same pick-one-loop-out-of-two box can serve as a simple effects bypass where one of the loops is a straight wire feedthrough.
In truth, an A/B/Y box *can* do a lot of this except that whatever is inserted after the A or B outputs may go to separate amps or channels. That, in itself, may be what some players need to do (e.g., clean rhythm goes through one signal path to an amp that complements rthythm playing, while solos go through another set of effects into an amp that best complements the tones you want for solos), but obviously this assumes one uses a multi-amp, or at least multi-channel, setup. For players who have more pedals than their feet can step on but only ONE amp a loop-select box is optimal