Why “voice of reason” is used in a sentence?

For instance, “I hate to be the voice of reason here, but even if you do successfully manage to acquire a goat, then manage to paint it with pink and green polka dots, and then manage to get the goat on the roof, someone is almost certainly going to call the police, report it, and get it removed before Joe gets home from work. How can a boy survive to be attacked from above by polka-dotted livestock? ”

A list of the 10 best/Fantastic/Nominal things to do in life../ ” “