When replacing a faucet, I was told that I should put some putty underneath the faucet. Why?

This is a good idea to keep moisture from getting under the faucet and into the cabinet below.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.