OK,
IN THEORIE you'd deviate from the middle knot up in light winds so you can pull in a little more and have more power on average and you'd pick the lowest knot to depower more, i.e. riding w/o your arms stretched out. The latter you can achieve with the trim strap as well,
and
PRACTICALLY if you go on the lowest knot rather than using the trim strap you can't undo that setting and might end up underpowered (happened to me, was slightly gusty and I thought it was stronger) and going on the upper knot I often pulled in to hard in light winds and stalled. So none of these other knots really ever worked for me.

Disclaimer: This is me. I might be wrong.

Steve