Initially, DOT is being staked to earn new issuance (“inflationary”) subsidies. It means that the supply of DOT will increase and stakers will capture the newly issued DOT. Generally, you will earn around 14% annually on your staked DOT, but it is subject to change.
Stakers will also capture fees from network transactions, and as Polkadot transaction volume increases, DOT stakers will earn more than just new issuance subsidies. DOT also gives stakers the right to vote on policy decisions on how the Polkadot protocol will operate and distribute treasury funds.
The main drivers of the DOT’s value could be more than transaction fees. Since DOT is required to lock up in parachain bidding, a new side chain will be required to procure and lock these DOT tokens in competition with other side chains. DOT-holders are also expected to be able to get early exposure to the assets of new side chains by lending their DOT for this bidding process.
Owning staked DOT is ownership of the Polkadot network, making DOT stakers entitled to setting/changing the rules of the Polkadot protocol.