Garbageman protects the bitcoin network from DoS attacks that can result from Libre Relay nodes relaying non-payment transactions, usually altcoin Ponzis.

Specifically, Garbageman is a node with a sane, more restrictive default policy that pretends to be a Libre Relay node in order to subvert LR's preferential peering mechanism, a custom feature LR uses in order to facilitate the spread of data spam on the bitcoin network.

Since data transactions are known for causing utxoset bloat and fee spikes, and since they compete with legitimate payment use cases, Libre Relay's preferential peering mechanism is a malicious exploit that makes bitcoin worse money, and in the worst case, prevents bitcoin from fulfilling its goal of becoming the world's dominant medium of exchange, perhaps permanently.

Since no one can stop LR nodes from peering with each other in this manner, the only defense is to beat them at their own game, which should be straightforward assuming there are many more GM nodes than LR nodes.