A chandelier needs a tall ceiling. The flush-mount fixture, and its close cousin the semi-flush, answered the plain problem of lighting a room without one. In solving that constraint, they produced some of the most useful lighting old houses have.
A constraint that improved the design
The flush mount sits tight against the ceiling; the semi-flush drops a few inches on a short stem. Both exist because the standard room, especially in the smaller houses and lower-ceilinged rooms that make up most of the housing stock, can't take a hanging fixture without someone walking into it. The designers had to light a room fully while barely leaving the ceiling, and the constraint pushed them toward compact, efficient, well-proportioned forms.
That compromise turned out well. Freed from hanging low in the room, the flush and semi-flush fixture concentrated on the job: a shade or bowl close to the ceiling, spreading light across the room without dominating it. The glass carried the period, opal, frosted, patterned, or hand-painted depending on the era, while the fixture stayed close and unobtrusive. Across styles, from ornate early-electric to spare mid-century, the flush mount adapted, which is why it shows up in nearly every period. Restored and rewired, it does the plain, essential thing it always did: fills a low-ceilinged room with even light and stays out of the way.
Salvager's Note: For a ceiling under about eight feet, a flush mount (tight to the ceiling) clears heads better than a semi-flush (dropped a few inches). Match the choice to your ceiling height before you fall for the glass.