why there is a need to find Molarity and Molality of a substabnce ?

1. Molarity and molality both are concentration units for a solution.
2. Molarity = $\frac{number of moles}{volume of solution in L}$
3. Molality = $\frac{number of moles}{mass of solvent in kg}$
4. Both of these units provide us the measures to calculate the concentration of a solution under different conditions.