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.
