2013 Touareg brake wear sensor

Just wondering do these need to be replaced?

If you have a check brakes warning on the dash, then yes it needs to be replaced when you change pads. They have a wire running through the plastic that makes it a closed loop and keeps the warning off. When the pad wears and wears through the sensor is cuts the wire giving you the light. They are not hard to replace.

As above but for what little they cost, why wouldn't you since they can be hard to extract from old pads?

They are cheap (approx $9 each). Replace all 4 when replacing the pads. If a wear sensor comes on, it means the conductive wire has broken because the pad life on that wheel is spent. Replace them all when doing pads.