Question:
Why is it necessary to turn off the water pump when washing a solid in a Buchner funnel?

this is when doing purification in science, or vaccum filtration

Answer: When you filter your sample/product in a Buchner funnel in which you need a aspirator/vacuum pump, you need to turn off the vacuum first before washing it with a wash bottle because when you let it turned on, the impurity which settles in the funnel might get into the distilling flask. You also impure the sample on the flask and there is an error if you want to weigh the precipitae/impurity that was in the Funnel