There is number of options: - video ads - app downloads - survey - app goals rewards
can they do all this things for announce their products and companies?
Yes, they can. And also they can do much better targeting using smart contract.
how can they use smart contract?
A smart contract can choose a level of users include completed awards and special requrements like country, genre e.t.c. The contract will use user's reputation and price depend on it
what is the main difference between a normal contract and a smart contract?