Yeast nutrient?

What are the benefits of using a yeast nutrient? Does it cause the yeast to produce fewer off flavors, or just simply help it consume the sugar?

As I understand it, it helps the cells when they are dividing by strenghtening the cell walls and building up reserves so that they can do their job more effectively which should translate to less off flavors

The benefits depend on a couple of factors, mainly the yeast strain, and the deficiency of nutrients in the must. If the must has enough stuff for the yeast to eat and grow on, then its not necessary. Nearly all yeast have different requirements that you can check out on the mfg website. My favorite example is RC212 vs EC1118. The first has a high nitrogen requirement, whereas the latter will ferment concrete slurry if allowed.

Redman is correct. It's basically "fertilizer" for yeast. It gives the yeast extra nitrogen. It's a good idea to use it if you're shooting for a high abv cider (starting with a high sg...maybe over 1.07?).
On the other hand, if you're starting with a lower sg such as 1.06 or less, I don't think it's truly necessary but wont hurt. I stopped using it because I try and get the yeast to poop out early without sorbate/sulfite and leave some residual sweetness. That is harder to do if the yeast are going strong.
So if your shooting for dry, nutrient is not a problem. Especially if you add lots of sugar to start.

It definitely helps ensure a fast, clean fermentation. I get an occasional stuck fermentation (at around 1.020 from an OG of 1.050) when I don't add nutrient, depending on the yeast strain (and of course the cider composition, but most of us don't have control over that). I then add 1/4 tsp per 5 gal and fermentation picks right back up. I've started adding nutrient preemptively to most batches since it also seems to cut down on the sulfur smell (usually dissipates but can linger a while).

Been reading and have a question. I know nutrient is cheap, but I'm trying to go a'natural. What would be a good home made nutrient? I am making a Apple cider and did 2 things. I boiled 2 table spoons of bread yeast and about 50 raisins chopped up and boiled them for about 5 min. The cider is doing real well with a high requirement yeast. Your thought?

Definitly appreciate the natural approach. You have the right idea with raisins although they don't "need" to be chopped up. The bread yeast on the other hand...you can do much better! If you're still in "experimental mode", I guess it's good to start with basics, but you'd be amazed what getting a good brewing yeast can do. I'd say an ale yeast like Nottingham or S-04 or 05 or Lalvin D47. Those are good ones to start with. That's just my suggestion but there would be many more options and suggestions from other folks. There's always old faithful Lalvin 1118 or 1116 but I don't use them anymore...because I've moved on.

I'm not brewing with the bread yeast. I boiled them with the raisins to kill them. Since most nutrients contain yeast hulls, I thought I would provide some also. The yeast I am brewing with is Lavin 212. Thanks for the heads up on not needing to chop up the raisins.

Lalvin RC-212 requires tons of nitrogen, probably 2x any of lalvins other yeast. Does it heavy with some pure DAP if you can.