The schwa is kind of an all-purpose symbol used to indicate an unstressed vowel.
Re: uses of the schwa
All vowels--in every language--are classified according to where the tongue is located in the oral cavity (i.e. inside the mouth). For example,
The vowel [i] as in "see" is produced in the front of the mouth: the top front of the tongue is used.
The vowel [u] as in "shoe" is produced in the back of the mouth: the top back of the tongue is used.
The vowel schwa is produce in the center of the mouth: the tongue is in an neutral position. Schwa sounds like the "e" in the word "the".
All vowels, when stripped of their place of articulation become neutral, which is represented by "schwa".
Vowels tend to reduce to schwa in unstressed environments:
tomato => t[o]mato (the vowel is stressed)
t_mato => t[_]mato (the vowel is unstressed) *the symbol _ = schwa
today => t[u]day (stressed)
t_day => t_day (unstressed)
Schwa has no place of articulation. It's neutral.
Click on the link below for more details:
In connected speech, about half of the vowel sounds become Schwas, apparently.