You might need the value of rents or leases on a property, but not the underlying value of the real estate. This is more complicated than it sounds, and isn't just a matter of how much a tenant is currently paying. A professional, well trained appraiser will examine rents in the market to determine fair market value of rents or leases. If tenants are paying below market value, they might be able to sublet at a profit. If paying over market value, there is a likelihood of moving on when the lease term is up increases. There are many things requiring the professional judgment of an appraiser that factor into the calculation.