When work with vectors using algebra, we add them to and subtract them from other vectors.
When we subtract one vector from another of same value and direction, we get a vector which as a zero size. However, to maintain that the result of operations on two vectors is a vector, we indicate that as the zero vector. It is for the convenience and consistency of the algebraic operations.
similarly, when a cross product is taken for two vectors which are parallel, the resultant needs to b e denoted by a vector/ That we call as zero vector.
the direction can be arbitrary bu the magnitude is 0.