To prepare sich solution, we would have to have a good measure of the solution added and of the final volume.
We may use a volumetric pipet to prepare it, but we won't prepare in a pipet, we would use it to measure something we would add.
A beaker or an Erlenmeyer flask don't have a good precision for volume measure, so they are bad to prepare the solution.
So, the best is to prepare in a volumetric flask, because its volumetric meaures is very precise.
