Line arrays put all the drivers in a vertical line (obviously) — it's the closest thing to a point-source, which would be a single full-range driver, or drivers mounted concentrically.
They're more "accurate" than old-fashioned designs where the drivers are in different places on the baffle; on these older designs the sounds come from slightly different locations and don't cohere as well into a single "image".
Line arrays also allow a narrower baffle — this reduces "edge-effects", diffraction, and certain other distortions — and their narrowness makes them less prone to resonate. They also have a taller, thinner appearance which is in vogue these days (Super Models are also tall and thin, and also in Vogue). The woofer(s), which must be bigger to produce deep bass, are usually put on the side(s) of the cabinet. This makes a line-array cabinet deeper. It also defeats the "line source" principle, but it doesn't matter so much with deep bass: they say bass below 100Hz can't be "located" by the ears. Though the smaller baffles are less prone to resonate, the larger sides do resonate, so internal bracing is needed.
Imaging is improved. An analogy would be two sharp eyes (20/20 vision) focusing to define a single image more clearly than two blurry eyes.
These are some of the arguments in favor of a line array. But your statement that they "sound so good" is a subjective opinion.