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Descartes was trying to provide a foundation for knowledge, and the existence of God is a keystone in Descartes' foundation for knowledge. Once it's established that God exists, it then can be established that clear and distinct ideas are true because if they weren't true, God would be a deceiver.

God, then, plays this essential role in Descartes' axiomatic philosophy: God exists; God is not a deceiver; therefore, clear and distinct ideas are true.

Right, but you have to build to God. If you don't "know" there's a world in the first place you can't have God the Creator. If all of everything is an illusion (not a lie necessarily, there's a difference) then so would be God. It works like all mathematical proofs, you've got to start with that which is so absolutely obviously patently true that it doesn't need tobe proven. The only thing you have absolute knowledge of is your own thoughts. You think, this is a given, this proves that you exist, from there more things can be proven. It's not until we have proven that there is a reality we live in that we can even begin to prove that said reality was made by God.