1) Because they form a generalization for prime numbers, and prime numbers are important.
2) Because they correspond to irreducible curves in geometry. For example, the curve y=x2 is irreducible, and indeed, the ideal (y-x2) is prime in [itex]\mathbb{C}[x,y][/itex]. On the other, the curve xy=0 is not irreducible since it exists out of the pieces x=0 and y=0. And indeed, the ideal (xy) is not prime.