Why: Arjuna acknowledges he has heard in detail of the origin and dissolution of beings, and of Krishna's imperishable glory, summarizing what the previous chapters accomplished before making his bigger request.

In life: it's often useful to briefly summarize what you've already understood before asking for the next, harder thing, it clarifies exactly where the gap actually is.

Teaching: before asking for a deeper explanation of something, briefly summarize what you already understand first, that summary sharpens exactly what you're actually still missing.