Believe the parent is referring to year over year increases, which are quite common in hot markets. A lot of landlords will jack up rents when it comes time to renew your lease, especially if they know there is room to squeeze you.

The first contract term is. The implication is by knowing your salary they can do a much better job guessing how high they can raise your rent the next year (or whatever) without you leaving.

At apartments organized by property management companies, the rent typically goes up every year. By providing your income, you give them a maximum they can easily creep the rent towards.