The sodium is just the conjugate base of the alcohol. You don't want to mix things like CH3CH2OH and CH3ONa, because then you might get a mix of ethyl and methyl reaction. So when you need these conditions (which are an equilibrium of alcohol and conjugate base for SN2 or base type reaction, utilizing kinetic control), you want to use the alcohol of interest (usually the connecting alkyl piece varies), and then the conjugate base to make sure you always get the same product and not a mixture.