We know that bhagavad gītā summarises the upanişads.
But we often forget that bhagavad gītā is a 700 ślökā: summary of the innumerable mantrā: of the upanişads. The bhagavad gīta was included in the mahābhārata by vēda vyāsa as a summary of the philosophies, intended for the informally educated. (The mahābhārata is more than just a history of kingly dynasties – it illustrates all the branches of knowledge using the lives and conversations of these sages, kings and gods. It is an exposition of the human condition.)
Serious students of Indian Philosophy would like to know all the ideas, thought processes, arguments, reasoning and insights behind the conclusions and techniques presented in the bhagavad gītā. So they study and discuss the upanişads.
These ideas were ignited in my mind by a conversation with my son.