1 : a thing of little importance; a very easy task
2 : any of various games involving the rolling of balls into scoring areas
3 : a short literary or musical piece in light style
The bard sat at the harpsichord and played a simple bagatelle while recalling the events of the day.
I think the point of this word is the musical one. Although I can see several reasons to use the other two meanings, it does help with atmosphere when describing any scene where music of a tavern or inn. Or, as above you can discribe a bard’s music as such. For the kind of music it represents, look up Francois Couperin - Les Bagatelles.