The idea behind using a stream to boost vaults and have people vote on it via gauges is to create an incentive for YFI holders to invest their tokens in the vaults. This could be done by allowing YFI holders to receive a portion of the profits generated from the vaults in exchange for their YFI tokens. This would create an incentive for YFI holders to invest their tokens in the vaults, as they would be rewarded for doing so. Additionally, the voting system via gauges would provide a way for YFI holders to have a say in which vaults they would like to invest in, as they could vote on which vaults they think will yield the most profits. This would ensure that YFI holders are investing their tokens in the vaults that they think will be the most profitable, which would in turn increase the potential profits of the vaults.