“I’m thinking about a larger mixer for my band and am curious about how I might use the matrix on Allen and Heath’s GL3300?”
The matrix is very useful for quickly creating mixes that are different from the main left & right mix.
While the possibilities are numerous, you might use it to control a separate set of speakers from the main left & right PA in the hall (e.g. speakers in another room), or you could use it to send signals to a recording device. Matrix outputs are usually fed from the left and right mixes and all of the subgroups (in the GL3300’s case, the stereo returns can also feed the matrix).