It is needed because the input stage need a very high value input impedance in parallel with the 1Megohm so that the 1Meg resistive value for the input of the scope will be maintained and also very low bias current. Active probes use a similar FET buffer. If you remove it the input resistance and capacitance of the IC opamp will be in parallel with the input resistor.
In case of high input voltage, the 10k resistor limits the current through the diodes. The 100pF capacitor bypasses the 10k at high frequencies to eliminate the frequency response rolloff that would otherwise be caused by the 10k and the capacitances of the diodes and the JFET gate capacitance.