Now that you’ve posted extra sections from the same piece, I am pretty certain that this is used as notation for a half-barre (or to be more precise, an incomplete barre). It’s slightly misleading, as you often don’t need to barre all of the notes within the brackets shown.
This is a rather ambiguous music notation marking anyway. It is dependant upon context and which instrument it is written for. To give some examples:
- for bowed strings, this bracket indicates a double-stop
- on piano this can show which notes are played by which hand
- on instruments such as harp, where chords are naturally "rolled" (arpeggiated), it tells the performer to play the notes without a roll, all at the same time
Furthermore, as Tim points out in his answer, this bracket has also been used to indicate the opposite of the last meaning above, to play the notes with a roll - although I haven't seen this usage.
Context is everything: because of the fingerings indicated in your excepts (the chord "shapes"), and as there are no other markings usually associated with half-barres (1/2C, 1/2CV, 1/2V etc.), this must mean use a half-barre, or incomplete barre.