Question

Why should you NOT turn off the tap of a water pump when you perform a suction filtration?

Answer 1

Because the flask is still under vacuum. The water already collected might fill the filtration vessel.

Explanation:

An example of a suction filtration setup is:

The passage of water thru the tap entrains air. Since your filter flask is at lower pressure with respect to the atmosphere, and connected by a hose to a flask with the water overflow, atmospheric pressure might back fill the flask with the water already collected in the overflow. You could always try this out as an experiment.