Being a "melting pot," and valuing individualism, Americans have less a sense of collective, cultural moral and ethical principals than say, the Japanese.
Religion provides a set of principles to live by: it tell people how you will likely behave in various situations, and tells them how you will will likely treat them and what your expectations are.
Given the limited cultural standards for principles in the US, folks are often interested in hearing a quick and dirty label that encaplulate the principles held by politicians, teachers, dating partners, roommates, etc. It provides a quick read about compatability and expectations in a wide range of situations.
So, in a principle-ambiguous society, labels like "Christian" and "humanist" can be particularly useful in social and professional contexts.