Re-hydrating is just that. You are re-introducing water to the yeast and bringing them out of a sort-of hibernation. This is a 15-30 minute process vs. 24+ hours from a starter. Also, re-hydrating doesn't increase cell count from what is already there.
With a starter, you're basically prepping/brewing a mini-beer to increase cell count and viability.
"There is no strong beer, only weak men"
"Pretty women make us BUY beer, ugly women make us DRINK beer" - Al Bundy
"Give a man a beer, he'll drink for a day. Teach a man to brew, he'll be drunk for the rest of his life."
Primary: Viking Metal, Berliner Weisse
Kegged: Oaked English Mild, Modus Hoperandi
Notable Empties: Oaked Black IIPA, BBK I, Red IIPA, Burning Bush, Apophis "The Destroyer", Vanilla Porter
On-Deck: Hobbit Ale, The Titan BW, Ale of Olympus