Zero (0) is used as a number and also as the numerical digit. Zero gives the additive identity of the integers, real numbers, and many algebraic structures. It is used as a placeholder for writing numbers.
Natural numbers start from 1, then 2 and so on. We cannot count backward with them. Integers, on the other hand, allows us to count backward. Zero winds up being a good choice for the number that should come after 1 as we count backward. Integers can go in both directions, to a positive infinity and also in the reverse direction to a negative infinity. Zero winds up being in the middle and separates 1 and minus-1 (-1). Then Zero becomes the first number of Integers.