ETH is the sole medium with which blockspace can be purchased within the Ehtereum network (i.e., the only medium in which transaction fees are paid).
It means that all ecosystem-wise value-transfer activities (native ETH and any tokens or assets) require ETH payment for the transaction
[1] = ƒ(blockspaceDemand, feeMarketPressure)
blockspaceDemand — measurement of the computational resources required to process transactions (directly tied to ETH's exchange utility)
feeMarketPressure — an abstract reflection of the competitive dynamics of transaction fee bidding (a direct value transfer mechanism)