Upanishads is created to give the self knowledge ( Atman-Gyana), therefore Upanishads are main texts for Vedanta philosophy.
Vedanta is philosophy which believes that Liberation can be attained by Knowing the Atman as the source and creator of this physical body and world.
The main purpose Upanishads were created is to negate the individual’s identification with physical body.
Brahman and Jivatma – Upanishads explains that this Jivatma ( Individual self) is actually Brahman, who under the influence of Maya considering himself as a limited body, and subject to birth and death.
Maya – Maya is a power of Brahman, due to which this world is seen.
Upanishads says that world has no real purpose, as it is creation of Maya and is falsely believed as real by Brahman. It is similar to a dreamer, who creates a dream world and see himself as a body in the dream and think dream world as real.
Because of this teaching of Upanishads, which comes in last sections of Vedas, all the rituals, actions , worshiping all acts lose their relevance, because they presupposes body as mandatory prerequisite and are performed within this false senses of reality.