By definition, "diastereomers" are geometric isomers that are NOT mirror-images. We could look at trivial cases of diastereoisomerism, i.e. "cis" versus "trans-2-butylene", which certainly have DIFFERENT physical properties..to potentially optically active materials the which have MORE than ONE chiral centre...
And thus a compound with two STEREOGENIC centres, could have "RR" or "SS" enantiomers...and these are diasteromeric compared to the "RS", "SR" set... The diastereomers should have DIFFERENT chemical and physical properties, including solubility, and this allows for the opportunity to separate the isomers, PRIOR to a final chiral resolution in which optical isomers are separated from each other.
