Rehydrating dry yeast prepares them to ferment the wort and ensures that as many cells as possible survive the initial pitching. I guess this is like ensuring viability.
From the Danstar FAQ page (
http://consumer.lallemand.com/danstar-lalvin/beerfaq.html):
"For the initial few minutes (perhaps seconds) of rehydration, the yeast cell wall cannot differentiate what passes through the wall. Toxic materials like sprays, hops, SO2 and sugars in high levels, that the yeast normally can selectively keep from passing through its cell wall rush right in and seriously damage the cells. The moment that the cell wall is properly reconstituted, the yeast can then regulate what goes in and out of the cell. That is why we hesitate to recommend rehydration in wort or must. Very dilute wort seems to be OK."
The FAQ goes on to say:
"We recommend that the rehydrated yeast be added to the wort within 30 minutes. We have built into each cell a large amount of glycogen and trehalose that give the yeast a burst of energy to kick off the growth cycle when it is in the wort. It is quickly used up if the yeast is rehydrated for more than 30 minutes. There is no damage done here if it is not immediately add to the wort. You just do not get the added benefit of that sudden burst of energy.
How do many beer and wine makers have successful fermentations when they
ignore all the above? I believe that it is just a numbers game. Each gram of Active Dry Yeast contains about 20 billion live yeast cells. If you slightly damage the cells, they have a remarkable ability to recover in the rich wort. If you kill 60% of the cell you still have 8 billion cells per gram that can go on to do the job at a slower rate."
This information is from Danstar, but I bet it also applies to Safale yeast. Even though the Safale package says it is ok to just sprinkle the yeast on the wort, they do give rehydration instructions on the Fermentis website (
http://www.fermentis.com/FO/EN/06-Ales/40-10_directions_hb.asp).