On p.249 of Freedman and Van Proeyen's Supergravity, the following is stated:
"Given the action of a supergravity theory, it is generally useful to search for solutions of the classical equations of motion. It is most useful to obtain solutions that can be interpreted as backgrounds or vacua. Fluctuations above the background are then treated quantum mechanically. The backgrounds that are considered have vanishing values of fermions, and are thus determined by a value of the metric, the vector fields (or higher forms) and scalar fields."
My question is: why do the fermionic fields have to vanish for solutions to a supergravity theory that preserve some (or all) supersymmetry?