Initially the ATOM is being staked to earn new issuance (“inflationary”) subsidies. That means that the ATOM supply will increase and stakers will capture the newly issued ATOM. Generally you will earn around 10.5% annually on your staked ATOM, but that can change. Since staking rewards are tied to inflation, read about how inflation and rewards are related here.
Stakers will also capture fees from network transactions, so as Cosmos Hub transaction volume increases, ATOM stakers will earn more than new issuance subsidies.
The ATOM also gives stakers the right to vote on policy decisions for how the Cosmos Hub will operate and distribute treasury funds.
The main drivers of the ATOM’s value could be more than transaction fees. ATOM-holders should be able to somehow extract value related to the “assets under management” that the Cosmos Hub secures (via products like DeFi). In a world of thousands of connected blockchains, credibility could be the scarcest resource, and the Hub aims to be the most credible. Owning staked ATOMs is ownership of the Cosmos Hub, entitling ATOM stakers to set/change the rules of the Hub. Read more here.