Rehydrate yeast first, or not?

I'm new to this.. fermenting my first batch right now (apple cider wine). I had a question about re-hydrating yeast. Does everyone do this? I was told to just sprinkle it on top and not bother with rehydrating. It's fermenting just fine. What's the advantage of rehydrating first. Also, why the high temperature of 100-105? If it ferments at 65-75, why would it matter so much that you have the 100-105 temperature?

I brewed my first batch by just sprinkling dry yeast on the surface and it took about a full day to show even the initial signs of activity.

You're always better off by making a starter wort a day of two before brew day to get the yeast active and multiplying. A simple wort like 10:1 (2 litre of water : 200 grams of DME) boiled for 15 minutes and cooled to room temp, pitch the yeast and cover with aluminum foil or an airlock in a large enough vessel to allow some room at the top. Pitch the starter into your full batch of wort at preferably no higher than your expected fermentation temperature and within a few hours you should (but, not always) be able to see visible signs of fermentation. This helps the yeast start out strong in that huge volume of wort.

I wouldn't try putting any type of yeast into 105 degree water, unless you're trying to mass murder billions of little beer makers!

That was the temperature on the package.

I am not sure why the 100-105 but, rehydration is meant to reduce stresses on the yeast so they sort of "hit the ground running". IIRC, when yeast hydrate from wort/must they must, obviously, allow sugar through the cell walls which can lead to some stress.

Not the end of the world but, not ideal.

Yeast mass murder will not occur below 140*F.

The packaging says to put it in 100 degree water, so I do. I follow what the packaging says because it is written to give you the best results, not the worst.

I notice that when I rehydrate the yeast, it starts activity a few hours sooner than if I hadn't. But if I buy something like pasteurized juice for the millionth time that I already know the SG and pH for and I'm not adding any sugar, since I'm not handling the juice at all, not even to sulfite, I'll pitch the yeast straight in to reduce contact. It will eventually start eating.

Strangely, my husband was working with a beer yeast last night that said not to hydrate it, but he decided to, so it got glopy, and the glops are now floating on top of the batch.

Point is - follow the directions on the packaging to create optimal conditions for your yeast, which yields happy yeast and a good ferment.

I used to simply pitch dry yeast and was fairly successful though I could see instances where the yeast experienced stress. I now rehydrate all my yeast with Go-Ferm in 100F-105F water. I have a lot better fermentation with the healthy rehydrated yeast.