What's the purpose of this resistor?
Marcus Müller was correct in the comments. I'm just rephrasing what he said here.

As the chip uses low current at DC, the impedance of the source through \$R_{\text{IN}}\$ is small, and there is only a very small voltage drop over the resistor (in any case, it will be a tiny fraction of the drop over the diode in series with \$R_{\text{IN}}\$) . 
However, when the device wants to draw high current spikes because it might switch an output and need to charge large output devices, the impedance through the capacitor is far smaller than through that resistor, so it will draw current from the capacitor instead of the resistor. This prevents high-frequency noise due to the current spikes of the IC to get into the power rails and couple towards other devices. 

This only serves to stop noise going from the IC into the external powersupply. It will not give as stable a voltage rail as with just the decoupling capacitor.

Sometimes the resistor is replaced with a coil, which will serve the same function but the DC resistance is lower (which can be needed for higher DC current drawing devices).