Obviously, for knocking on the table at a club... but that's not what you really want to know. Why would people knock a mallet on the table?

To indicate approval... kind of like applause

To keep time to the music... like tapping your toes, but louder

To get another drink

They were frequent night club accessories in the mid 20th century. I haven't seen them in use in 30-40 years, so they are nostalgic souvenirs if they have the Club name on them. Somewhat like having the club photographer take a photo of you and your companion.