I'm not sure what will end up being the main utility for the Strat coin, but these are some of the things it will be used for:
1) Funding sidechains. In order to use a sidechain, Strat must be sent to the sidechain federation. In return, sidechain coins will be released to the sidechain user. The Strat is locked up until the sidechain coins are sent back to the federation, at which point the Strat will be redeemed by the user.
2) As gas for smart contracts. Stratis has a similar gas solution to Ethereum in that the gas is the coins of the chain to which the smart contracts are deployed. So in order to use a smart contract which is deployed to the Stratis mainchain, the user must spend some Strat as gas. Similarly, to use a smart contract deployed to one of Stratis's sidechains (which is where most smart contracts will be deployed) you must spend some sidechain coin as gas. Since this sidechain coin is pegged to Strat (via the cross-chain relationship above), this translates to utility for Strat itself: you need Strat in order to receive sidechain coins with which to use smart contracts (UX of this process need not be so technical, this is just the underlying economy).
3) As received funds through the ICO Platform. The Stratis ICO Platform allows investors to invest in an ICO using all of the major cryptocurrencies (something like 50+ of the most liquid coins). Any of these currencies which are not Strat or BTC will be automatically converted into Strat via Changelly. Changelly automatically buys Strat from exchanges with the other currencies. Projects can receive up to 400 BTC as well, which is to help alleviate the impact on the Strat markets of projects liquidating funds to pay for development post-ICO. Besides this BTC, the projects running ICOs/STOs through the ICO Platform will receive Strat.
Then there are specific use cases which are periphery to the core platform. For example, Strat is used to perform attestations in Stratis Identity. Those kinds of utilities are for applications built on top of the Stratis Platform, so who knows what utilities could be dreamt up, which is why I'm not really including them in what I think of as the core utilities for Strat.
May as well take a punt at what could be the main use case for Strat. If I had to make a guess, I'd probably go for "funding sidechains". The smart contracts will be deployed to a PoA sidechain network which is coming out this month. To use these smart contracts, Strat will have to be locked up in return for the sidechain coins. These coins will in turn be used as gas for the smart contracts. Even the token issuing contracts alone could see a lot of Strat being used for funding sidechains.
if the ICO's that are built on strat see significant advantages compared to ethereum, I'm guessing we'll see a massive usecase in the stratcoin that way, but like you said i think all of them will play a part in the success of the token