I’ve been using Glyphs for about five years now and in all this time, I’ve mastered lots and lots of things, but I am still completely bewildered by placeholders.
They seem to me to be most helpful for the kerning of letters, but in order to kern a pair, for example, you have to put your cursor between the two characters you want to pair, and then your placeholder changes.
The placeholders are meant to show the current glyph in a different context. So you can do ‘nnxnn oo/placeholder/oo’. When you place the cursor in front of the x, the placeholder will show the x. So, as Rainer said, it is more useful for spacing than kerning.
(Note: I recently received some minor reports of misbehaving, those are probably due to some changes in the GlyphsApp API. If you encounter some, please report. The more reports, the more I’m motivated to work on it and fix the problems. Since it’s open source, it’s not too easy to invest valuable time if no one cares anyway =) )