What are the resistors bridging connections between devices generally implemented for?
These resistors indeed work as voltage dividers.  This is done for level-shifting the logic signal from 0-5V to 0-3.3V .

The snippet in the O.P. doesn't show it, but I suspect that the μC is powered with +5V in this schematic.  RFM128 is powered with +3.3V, and it isn't 5V-tolerant, probably.

Notice also that only the outputs from the μC (SS#, MOSI, SCK) go through voltage dividers.  The input to the μC (MISO) does not go through the divider, because it has 0-3.3V levels, because it's generated by an IC powered with +3.3V.