Docutils difference between text and rawsource in nodes.Text?

Question

When creating new nodes such as nodes.title, one needs to provide two strings:

1. rawsource
2. text

Often I see this:

nodes.title('foobar', 'foobar', ids=[id])

Or sometime...

nodes.target('', '')

I don't understand why this repetition, and why it is useful to have or not have twice the same value.