Why is it necessary to express the concentration in molality rather than molarity?

I would think it's a matter of ease, not necessity.  The molality (which measures the weight of the solute) might be more relatable to someone than molarity (which measures the moles).  Those who aren't familiar with the mole or with the conversion between weight and moles would be more comfortable with molality than morality. Of course, this is just my opinion, I don't think it's "necessary" to express one over the other.

What are the colligative properties we usually think of?  Osmotic pressure, boiling point elevation and freezing point depression.  With osmotic pressure, you never change temperature, but with the other two you are going to have to freeze or boil it.  Now assume we make the solutions at 20 degrees Celsius to be 1.00 M in a volumetric flask.  We carefully adjust the solution exactly to the mark.  Now we start to heat or cool, what happens?  The volumetric flask is like a big thermometer, the level of liquid in the neck will go up or down, and our 1.00 M is shot.  If instead it is 1.00 m we have used one mole of solute and one kg of solvent.  What happens if we heat or cool this, we still have one kg of solvent, and consequently 1.00 m  Molality is independent of temperature while molarity depends on temperature because of the expansion or contraction of the solution.