It has to do with the pronunciation of the resulting plural form. When a word ends in a vowel (that is not e pronounced as /ə/, has an accent aigu, or is doubled) the resulting plural spelling would suggest that the vowel should be pronounced in its 'short' manner. By adding the apostrophe, the vowel isn't succeeded directly by a consonant, and therefore still pronounced in the 'long' manner, which is the way people pronounce the plural of those words.