I think jno has the heart of it.
Not sure Naomi. Atheism obviously only represents a narrow facet of a persons worldview - one defined entirely by that persons response to religion or a deity. Humanism represents a much broader definition and acts as a code for a set of cultural and ethical values too.
There is a lot of discussion about another new descriptor for atheists with a social justice agenda - Atheism+ - but this is still an evolving definition.
http://www.humanism.org.uk/humanism
http://freethoughtblo...ghts-on-divisiveness/
Humans loves their stereotypes - it fits into the human need for a narrative, and allows decision making based upon a set of assumptions about how that individual might act, what common values you might share.
So when people define themselves by their religion - Jewish, Christian, Mormon, JW, Muslim etc - those labels offer some insight into a presumptive set of cultural and social values. I don't like that form of thinking myself - I think it lazy and misleading - but it is deeply ingrained.
So,the term Humanism offers some context to someones presumptive social and cultural values, and may actually serve to reassure those faithful who see atheists as having no ethical or moral framework.