Yup; and the company is making a bet that you produce more than you cost. Sometimes employees generate a lot more value than they cost; the company usually keeps most of the winnings there, and only gives you enough to stop you going somewhere else. That is, the competitive pressure on employee wages is much higher than the competitive forces within most companies that control whether they get to keep the windfalls from particularly efficient employees.
I'm not sure I've ever had a job that outlined my specific duties in the contract. It usually says that I am to work X hours a week.