Coronella Austriaca as we know have duvernoy glands which produce mild toxins. But I fail to see why a snake which uses constriction to kill its prey would evolve to have a sort of venom gland.

What is the need and purpose of these mild toxins?

Some snakes like the Boomslang produce potent venom from these glands and so the effects of it would be beneficiary. The only thing I can think of would be that the Smooth snake comes from a family of venomous snakes which has evolved to use constriction instead and the venom glands have been reduced but im not very well learned in taxonomy.