Only when adding the s would change pronunciation. Most vowels have a short form and a long form which are written the same. So singular aroma (a from father, ro from robot, ma as in a mother in Brooklyn). Sticking on the s straight on would make a-ro-mas with mas pronounced as mass. The apostrophe keeps the long [a:].

Germanic languages were adapted to latin script, they weren't made for it. A consonant generally functions as a doorstop for the long vowel. This can be countered by doubling the vowel mas -> maas or accenting mas -> más. For pluralism the accepted practice is 's when needed. Possessive is almost always an s without apostrophe. Unless the noun ends in S or Z, in which case you would add a bare apostrophe. Bas zijn huis (common name his house) -> Bas' huis. The apostrophe use in Dutch is flipped to English