Vaults are used to earn rewards. Tokens such as imUSD or Feeder Pool tokens can be deposited to earn additional MTA rewards. On Ethereum mainnet two versions of these contracts are deployed. BoostedSavingsVault is used for single token rewards (MTA) and BoostedDualVault is used for 2 reward tokens. However, only one reward token can be used for the BoostedDualVault as well. The Boosted in the name implies that this contract reacts to the staked MTA balance and receives a boost from that.