Meiosis is reduction division, so each gamete has half the normal complement of chromosomes. Therefore, when fertilization occurs, half of the chromosomes come from the mother (in the egg) and half from the father (in the sperm). That gives the zygote the normal number of chromosomes, the same as the adults from which the gametes came.
If meiosis did not occur, fertilization would cause each cell to have twice the normal number of chromosomes. Then, the next one would have four times as many.... soon there wouldn't be room in the cell for anything else!
Seriously, some major problems can be caused in humans if meiosis does not occur normally, if even one chromosome does not separate properly. It's called trisomy, and it means the person has three copies of a chromosome instead of the normal two... Downs syndrome is caused by trisomy of chromosome 21.