Why is molality a useful measurement?
Molarity is dependent on temperature, since the quantity of the solution is based on volume, and volume is a function of temperature.
The mass of a substance however is independent of temperature; this is what makes molality useful in a variety of situations that involve a drastic pressure or temperature change, which can change the solution's volume.
In such situations, the molarity would change, but the molality would remain unchanged. Thus, molality is often used in comparing and determining a solution's colligative properties.