In our local music community, players and listeners generally draw a sharp distinction between "tunes" (instrumental only) and "songs" (vocal, accompanied or not). There is a parallel dichotomy between "Trad" (instrumental) and "folk" (vocal), but the edges are evidently not as clear and crisp as in the first case. Thus, a "trad" band is one which focuses on the instrumental music, while a "folk" band focuses on the vocal music.Even the most die-hard instrumentalists have come to recognize that local audiences easily tire of strictly instrumental music, irrespective of virtuosity, mostly, I think, because audiences relate to the music differently than players. Listeners might know the name of a small number of tunes, and actually recognize them when played, but most are hard-pressed to distinguish between a jig, a reel, a hornpie, a waltz, etc, etc. Players, on the other hand, are more apt to become enthralled by the technicalities of a given piece, which is entirely appropriate.
With vocal music, the performers engage the audience in a different way, and perhaps at a different level(s). My guess is that whether your listener plays music or not, they can, and often do, relate to the language element of a song, which opens the door to an expanded appreciation for the non-musicians in an audience.
The local paper here commented about our group once by saying "there's none of that airy-fairy stuff here" (please, don't throw things; it was their expression, not ours). I viewed it as a recognition that strictly instrumental music in a seisiun begins to sound "all alike" after awhile to non-musicians, whereas, with vocal music, you don't have to be a musician to understand differences because you can follow the lyric, and indirectly, the music through the lyric's connectedness to the nuances of the melody.
Maybe Tommy Makem said it best. At the close of a show here a year or so ago, he commented that he was glad to see the revival of interest in Irish traditional music, "but we must preserve the song tradition as well."