Originally Posted by KIAKillerXJ
In a starter you are propagating the cell count.
Rehydration you are just preparing the yeast before pitching so that less die in the initial pitch into the wort
In addition, the dry yeast have a crap-ton more yeast cells active when rehydrated and pitched compared to just what's in the little vial of liquid yeast. That's why you have to build up the liquid yeast into a starter, but the advantage is that you have a wider selection of yeasts available. You even have a whole bunch of commercial beers you can harvest yeast from their bottles with a starter; I did that for the first time this previous week, pitched it yesterday, and 16 hours later my airlock is bubbling 2 or 3 times per second.
I feel like my creative spirit will eventually turn towards using more liquid yeast and playing around with starters. Then the dry yeast is great to have on hand for just in case circumstances, or for the quick extract batch here and there.