The greatest good for the greatest number.

Use=utility. Do what is most useful.

Utilitarianism is an effort to provide an answer to the practical question “What ought a man to do?” Its answer is that he ought to act so as to produce the best consequences possible.

A philosophy from the late 18th- and 19th-century English philosophers and economists Jeremy Bentham and John Stuart Mill that an action is right if it tends to promote happiness and wrong if it tends to produce the reverse of happiness—not just the happiness of the performer of the action but also that of everyone affected by it. The greatest good for the greatest number. Such a theory is in opposition to egoism, the view that a person should pursue his own self-interest, even at the expense of others, and to any ethical theory that regards some acts or types of acts as right or wrong independently of their consequences.

Utilitarianism explained

Why is it part of liberalism?

Because it's rational answer to questions of ethics and politics. Behaving well does not depend on magic, superstition, religion or ancient traditions.