These stacking headers let you extend a board on top of existing headers, while allowing a next level to "stack" on top. Perfect for JeeNode ports (6-pin) and for the PSIX header (8-pin). They are also very common with Arduino "shields".
The 6-pin (10-pack) variant is the most common one, for use with JeeNode ports.
The 8-pin (10-pack) variant can be used for the PSIX header, and of course for Arduino shields.
Note: these headers are not made from long strips cut into 6-pin units, but genuine 6-wide and 8-wide connectors with accurately machined smooth edges.