"I do, don't I?" What is this type of phrase called, and what is its function within speech?

Question

This kind of saying something then asking if it isnt so, seems on the surface, wasteful and pointless.

• "I do (X), don't I?"
• "I (do) (X)."
• "Do I (X)?"
• "Don't I (X)?"

Examples:

• "This is delicious food!"
  "I do make good pasta, don't I?"

• "This is delicious food!"
  "It is, isn't it?"

• "You're a good friend."
  "I am, aren't I?"

On the surface the extra phrase seems to gain nothing and be a contrition with little value. The replies could as easily be rephrased, with possible alternatives

• "This is delicious food!"

  1. "I usually make good pasta."
  2. "Yes, I'm good at it."
  3. "I do make it well; I'm glad you think so."

• "This is delicious food!"

  1. "It is."
  2. "Yep."
  3. "Yes, and did you like how the basil tasted?" (Or some other comment to engage dialogue)

• "You're a good friend."

  1. "I care about you."
  2. "I like to think so."
  3. "Yeah." (Optionally: But you're worth it)

But the reflexive phrasing is so ubiquitous it must have value in human communication even if not having value within a strictly logical interpretation of language.

What is the name of that kind of specific structure, where something is asserted then questioned in the negative, rather than either just asserted, or just questioned?

What is its actual functional impact, which makes it so used? (Alternative question, how does phrasing it that way enhance impact?) If say, "politeness" or "impact", what about that structure makes it seem so much more polite or impactful to a listener that it's worth the contortion?

Answer 1

These would be rhetorical questions. The second speaker does not intend for the first speaker to answer the question with anything more than a nod.

Depending on the additional context, it can be used in two ways. The first is to allow the speaker to modestly agree with a compliment that they already believe is true.

"This is delicious food!" "I do make good pasta, don't I?"

In this case, the speaker knows they make good pasta and are acknowledging it now that the other person has complimented them.

The second is if the speaker has made a self-realization based on the feedback.

"You're a good friend." "I am, aren't I?"

For this case, we can imagine a scenario where the second speaker was doubting that they are a good friend. By confirming what the first speaker states, they are boosting their own confidence. The question is one they are more asking themselves than to the first speaker.

Answer 2

This kind of saying something then asking if it isnt so, seems on the surface, wasteful and pointless.

Your opinion seems to go against that of the 1.5 billion people who have English as a first language...

In your examples, "don't I?"; "isn't it?" and "aren't I?" are all tag questions.

These are used to keep conversations going in some way, or to make assertions/denials, or look for support.