Marvin has all activated abilities of creatures you control that don't have the same name as this creature.
Marvin copies all activated abilities of your other creatures, so you assemble a board where Marvin becomes a Swiss Army knife of activated abilities. You ramp into Marvin early, deploy creatures with powerful activated abilities, then untap him repeatedly to abuse those abilities or assemble an infinite combo. As a colorless commander you can build it in any color identity, but most lists go mono-colorless or add a single splash for tutors and protection.
Colorless identity means it slots into any deck and is castable off any mana, including artifact ramp
Turns a single untap effect into an engine, enabling many two-card infinite combos
Cheap 2-mana commander that comes down early and gives repeatable value
Flexible toolbox: every creature you add hands Marvin a new mode
Heavily commander-dependent; Marvin dying repeatedly stalls the whole plan
Needs a critical mass of creatures with activated abilities to do anything meaningful
Colorless shell lacks access to premium interaction, board wipes, and counterspells
Activated abilities still cost mana/taps, so you can be mana-starved without untap enablers
Vulnerable to graveyard hate and creature removal that breaks combo lines
Legendary Artifact Creature — Toy