the "open" components determine the average DC output value. SInce there are series capacitors, the device DC level is stripped off, and you would only have an AC coupled signal at the output uniless you populate the "open" resoistors with the proper value. i.e. you would have a signal like perhaps +/= 0.5 volts, which would NOT be compatible with driving a typical digital gate.
Since there are all sorts of average DC levels for the various types of digital gates, and for their operating Vcc, they give you the option of dialing up the DC level you want.