Once per turn, when a Machine-Type monster is destroyed by battle and sent to your Graveyard: You can Special Summon 1 Machine-Type monster from your Deck that has the same Attribute and less ATK than that destroyed monster in the Graveyard.
Can only activate effect when a Machine monster you control is destroyed by battle.
Cyber Dragon monsters are all LIGHT Machine-type monsters so you can use Machine Armored Unit to cycle them out of your deck. With support like Cybernetic Fusion Support, Cyber Dragon Repair Unit, etc. they can make use of the Cyber Dragon monsters in your graveyard. Chimeratech Over Dragon can be summoned with a huge attack stat with the help of Machina Armored Unit’s searching.
Ancient Gear monsters have similar attributes and are all Machine-types. Because of the many high-attack monsters they have. You can easily start Special Summoning monsters with Machine Armored Unit when a monster like Ancient Gear Reactor Dragon is destroyed.