Simple keywords for definitions can make grepping for stuff much easier. Given that shell allows other forms, it isn't that helpful in practice. I still use it because my eyes find start-of-definition faster with common patterns that are not just special-chars. YMMV.

Edit: I usually don't care for portability, and just use bash. Shell is so ugly already, I refuse to write my personal stuff in portability hell mode. Rather level up my Civilization skills instead.