There is a "WHEN do" - part of this, and a "WHY do" - part.
The WHEN, I can answer - the WHY is just speculation.........
In my expetience, Microsorium develop plantlets mainly when stressed and leaves are at their last. Young and healthy leaves usually do not develop plantlets. Leaves with plantlets soon de-attach and flow off. Even leaves that broke off, will often develop plantlets if left floating around.
- this MIGHT give food to thoughts about why. A plant not really thriving at particular place, or a leaf not able to sustain itself, by this method invest energy in identical offspring, that can float off to settle somewhere better suited for growing.
- and yes; Microsorum do develop PLENTY of spores, growing emerse.........but can actually develop plantlets, too.......