Molality is used for those people trying to understand how the melting point or the boiling point of water or any other substance, changes when they add a given amount of salt or any other impurity to one kilogram of this pure substance. They don't care about the volume of the obtained solution. They will not keep the solution once obtained. As an example, they just want to check if it is correct to say that adding $1$ mole of a solute to $1$ kg water lowers the freezing point by $1.86°$C, whatever the nature of the solute.
Molarity is used for another group of people. They don't mind about the process of dissolution of a substance in a solvant. They don't care about the volume of water to be used to make $1$ liter of the solution. They just want to know the composition of the solution they have to use, i.e. how many moles (or grams) of a solute are present in a given volume of this solution. If for a given use, they must have a solution containing exactly $9$ g NaCl per liter, they don't care about the way of dissolving the salt. All they need to know is whether this solution can be used for making an injection, or any other use.