why is it necessary to make up to the 250 ml mark while during dilution?
You really need to state your questions a little clearer; however, I think I know what you mean in this problem.
You always make up to the mark or add up to some final volume in dilutions. When using a solid or a liquid to make a solution, you always want to end up with the total volume. If you add solute to a set volume, then the total volume changes. Two examples, 500 mL ethanol + 500 mL H2O produces a total volume of about 950 mL whereas if you add 500 mL ethanol to a 1 L volumetric flask and make to the mark with distilled water, then you KNOW the final solution is 1000 mL. A second example is adding 100 g NaCl to 1000 mL water. In this case the total volume is more than 1000 mL. In this case you add the solid NaCl to the 1 L volumetric flask, add some water, swirl until all of the solute is dissolved, then make to the make with distilled water. Then you KNOW the final volume is 1 L.