Normally these clamps are used to protect inputs against ESD, like it's shown here:

http://www.chrontel.com/pdf/an38.pdf

Here the same can happen: If no big capacitance is connected from Vcc to ground, Vcc can dramatically rise and destroy other circuitry being connected to it! So, in any case, directly at the cathode of diode, which shunts ESD-energy too Vcc, there must sit a 100nF cap to ground. This is not big enough to totally absorb the whole ESD-event, because the capacitance relation between body capacitance of arround 100pF and 100nF is only suited to suppress the ESD-voltage to about 1/1000, which yields 8V for a 8kV spike. But, it slows down the speed of ESD-event and makes other caps sitting at Vcc available for also absorbing the ESD-energy. Means, if in some distance 10µF capacitance is connected to Vcc, then the overvoltage due to the ESD-event will only be about 100pF / 10µF * 8kV = 0.08V.

An advantage of this Schottky diode clamp for ESD-protection compared to other measures is, that it adds only minimal stray capacitance to the signal, which is important for certain high speed applications. But nowadays also low capacitance transzorbs are available.

Kai