Well, basically we need to hear the "S", by hook or by crook. :D
Words which end in an unvoiced consonant just need the "s" (books).
Words which end in a voiced consonant get a "z" sound (eggs). It's much easier to say.
Words which end in anything like a "s", "z" or even a "sh" or "zh" sound get the /IZ/ sound... otherwise, we simply don't hear it (gauges, bushes, masses, sentences, fezzes).