Thematically, I think people get too hung up on the "traveling performer" concept, when bards can be so much more than that. The historical origin's been mentioned already, but there's also a lot of examples of bards in mythology that fit a variety of unique concepts but are still pretty distinctly "bard" in my opinion:
Sirens of Greek mythology used songs to charm men (Lore Bards of the sea). Odysseus himself would have been a Valor Bard more than anything; he fights, he leads, he sneaks, he deceives, he charms, and he's basically the best storyteller in the world. The Pied Piper of Hamelin used music to catch rats (and then children when he didn't get paid); fits thematically with nature, plague/necrotic spells, or possibly religious fanaticism (the tale is said to symbolize the Children's Crusade). I'm not sure where it comes from, but the "demon fiddler" is a concept I've seen a lot in media, and Tiefling Bards fit it perfectly.
On another level, I feel that D&D is about making giving cool powers to any sort of mundane concept in any given society, and art is pretty central to any culture. Lots of historical wars have involved the use of war drums for morale/marching order, and D&D's bard magic enables that role to provide a more tangible benefit than "we believe in our cause slightly more". Court jesters had a lot of power through their ability to say anything with impunity; they could mock kings and be the comedian who says what everyone's thinking, and the bard class is perfect for that mixture of performance, acrobatics, and wit. I'm personally mulling over a slimy "Hollywood Agent" concept for a character who builds reputations for his clients (adventurers) to get them higher-paying gigs (quests), and then he takes his cut; I can think of no better class for this than Bard, telling far-fetched tales of his clients' exploits in taverns.