The Machine Addon Extender is used to increase the number of available addon slots for a machine.
By itself, it does not provide any bonuses, but it lets you place more addons than the machine's base quality would normally allow.
Expansion
Extenders must be attached directly to the machine or to another connected extender. The maximum number of extenders a machine can support is determined by its Machine Core quality. Higher quality cores allow more complex and layered addon setups.
The machine quality can be viewed in the top-left of the machine UI when hovering over the machine core icon.