If your fingers would otherwise be touching the metal screwdriver blade, then it will provide some insulation. It will also prevent the shank of the screwdriver blade from making a short if it hits a live wire.
I am sure you would not intentionally be putting a screwdriver into an appliance with the power on, so this is presumably belt and suspenders in case there is power in a circuit that you did not realize was present and missed in your testing.
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