With a FFP scope, the reticle moves in and out as you change your magnification. It keeps your reticle the same at any magnification. In tactical terms, it subtends the same at any magnification. So the spacing inside your reticle does not change. The 1.0 mil line is ALWAYS 1.0 mils at any magnification.
With a second focal plane SFP, the reticle appears to stays the same size as you change your magnification. But, how much the reticle subtends changes with magnification.
For example, take my favorite scope, a Nightforce NXS 5.5-22 with MLR reticle (see link below). This is a SFP reticle, and the reticle is "set" at 22x, which is max magnification. The first short hash mark is .5 mil and the next full mark is 1.0 mil at 22x. Now, if I dial down to 11x, it's half power magnification. Since my image just got twice as far away, the reticle now subtends twice the value. So at 11x my first hash is now 1.0 mil, and the second full hash is 2.0 mils. And it's 4 times on 5.5x power.
So, is a FFP scope needed to hunt, No, not at all. But it can be helpful if you learn how to use your reticle. The FFP vs. SFP is always a big debate like a Ford vs. Chevy. I prefer the SFP scopes, since I have a finer cross hair to aim with on my tactical scopes. But I do have several FFP scopes on my hunting rifles, and it makes zeroing and using the reticle to hold for elevation at any magnification.
http://www.nightforceoptics.com/pdf/MLRSheet.pdf