During precipitation (before the washing step) we use 100% ethanol because we add a volume that corresponds to 2 - 2.5 times the volume of the solution being extracted. This results in a solution with an ethanol concentration around ~70% (e.g. 300 uL DNA-containing solution + 700 uL PA ethanol). This is the ethanol concentration required for DNA precipitation. During extraction, at no time do we really have 100% ethanol in contact with the DNA. Ethanol concentration higher than 75% during this step can cause further co-precipitation of contaminants, and more than one wash step would be needed to remove it.

Then, to wash the pellet, we discard the supernatant and add more ethanol, but this time we use 70% ethanol. Since the amount of liquid left in the tube after discarding the supernatant is very small, the resulting concentration of ethanol in the tube will again be ~70%. This concentration is sufficient to solubilize any remaining contaminants, while not solubilizing the previously precipitated DNA.

After centrifugation, excess ethanol is discarded and what remains in the tube is left to dry at room temperature or in a concentrator (e.g. speedvac). Here, if the ethanol concentration were too high (100%), the pellet could dry out too much, making it difficult to be re-suspended. Also, as I said before, high concentrations of ethanol facilitate the precipitation of contaminants, and we don't want that!

;)

Ref:

https://bitesizebio.com/2839/dna-precipitation-ethanol-vs-isopropanol/#:~:text=DNA%20precipitates%20in%2035%25%20isopropanol,2%2D2.5%20volumes%20of%20sample.

https://pubmed.ncbi.nlm.nih.gov/32066262/ - page 8, first paragraph