What are the benefits of venturing into the Nether?
These are all good answers, but they seem to be tailored to more advanced Minecraft players, and you wanted a KISS answer. (Keep it Simple Stupid)
The Nether is neccesary if your ever wish to craft potions, slay any of the bosses, and\or have access to some other unique things.
The Nether can also be convienently used to travel great (overworld) distances in a short amount of time.
While @Buhb's answer is tecnincally correct, villagers are random, and the Nether is gauranteed.
One key point... if you can't buy the Eye of Ender from a villager, then the only way to make it is with blaze powder crafted from blaze rods in the Nether. In short, no Nether == no way to get to The End.
Also, you won't buy wither skeleton skulls, nether stars or beacons. Mining gold in overworld (even in Mesa biome) will never yield anything close to what a good zombie pigman farm can produce. And your brewing stand won't be of much use without blaze powder to fuel it, and you won't get it from trading either.