Why are resistors used in a Zener clamp circuit?
In case input probe is connected to a point with a higher voltage than zenner's, it will draw a lot of current from that point if it is not limited by R1. Without R1, taking a measurement could break the circuit which is tested, the probe, or even the acquisition board, in case of a high voltage.

R2 may be there to protect high voltages which come back from acquisition board, maybe in case of malfunction.