Good point, there are two purposes to token on Ethereum chain:

1) Proof-of-Stake systems need something to stake on, in this case it will be the initial tokens on Ethereum chain.

2) The community wants to ensure a fair launch, so that everyone gets a fair chance to get a stake in the upcoming blockchain, while it is under development.

This is similar to main other chains that first launch as a token on Eth and then later move over to the new blockchain.