Ethanol has higher dielectric constant than water and will thus "soak-out" the water from nucleic acid and leave it dehydrated. Now positively charged ions in your solution can access nucleic acid and form salts.Without Ethanol (or propanol) positively charged ions don't stand a chance at accessing phosphate groups in DNA because they're surrounded by water molecules.

Depending on your approach and extraction protocol ethanol is added to help precipitaion of RNA/DNA molecules out of the aqueous solution, thus allowing their collection as pellets after centrifugation.

 EtOH needs to obtain the salting-out (precipitation) of the DNA/RNA  from a solution which must containg 0.3M NaCOOH or other salts

http://bitesizebio.com/253/the-basics-how-ethanol-precipitation-of-dna-and-rna-works/

Ethanol has higher dielectric constant than water and will thus "soak-out" the water from nucleic acid and leave it dehydrated. Now positively charged ions in your solution can access nucleic acid and form salts.Without Ethanol (or propanol) positively charged ions don't stand a chance at accessing phosphate groups in DNA because they're surrounded by water molecules.

EtOH is used to wash DNA to remove salts as explained by Dr. Lisnic. Also you can store your DNA in EtOH at -20C becuase DNA do not dissolve in EtOH.