Why can positive statements end with a negative question and vice versa?

I had a hard time phrasing the actual question title—hopefully this doesn't mean it's too subjective—, but I'm curious about why positive (or negative) statements can be terminated by negative (or positive) questions:

• You do like apples, don't you?



• We can't all be rich, can we?

Why is it (arguably) common to turn a general statement into a question like this when the intent remains the same? Is this a common use of a rhetorical question? If so, what is afforded by not simply making a statement without turning it into a question?

Answer

As a rhetorical device, I argue that it is a leading question.

... a leading question is a question

that suggests the answer or contains
the information the examiner is
looking for.

In law, a leading question should be avoided. I don't think that is necessarily true in informal (or even formal?) writing.