How important the EDTA in the sample buffer? and why is it needed?

Chelate metal ions, prevent DNA and RNase reactions, and reduce nucleic acid degradation.

Good morning, its function is protease inhibitor. If the sample is free of proteases, it can be ignored.

Thank you for the answer Jose.

I also have another question regarding the stacking gel preparation: According to the recipe, the total volume is 3 ml which contains 400 ul of the stacking gel buffer. According to the protocol, this buffer was prepared as 4X, which equals to 750 ul/3 ml stacking gel. According to the stacking gel recipe, the final concentration of SDS is around 0,05 % and Tris is around 0.06 M. Also the acrylamide concentration of stacking gel is 6 %. can you please advise on the stacking gel recipe?