Increasing at the edge


I have a question about increasing 1 st on each edge (right and left) on knitting. I understand that I can use M1L and M1K but I’m not sure which one I should use on which side. I understand that M1L has a left slant on the twisted stitch and M1R has right. I also understand that if I was doing paired increase with an axis as in a sleeve I should be using M1L for left side of the axis and M1R for the right side so it would be a mirror image. What I don’t understand is when I need to do M1 on each side of flat piece as in sweater front or back. One of my books says I should use M1L for Right edge and M1R for left edge.
This website also suggests that. But if so I want to know WHY.
Can somebody explain it to me why use one rather than another?

Thank you,

You use M1L on the right edge because it puts the new st leaning toward the left or center of the piece, and the M1R on the left edge as it’ll slant toward the right or center of your knitting.