A haploid cell is represented by the symbol (n) and represents the single set of chromosomes contributed by a sex cell or gamete to a future offspring. Therefore, a sperm or egg cell is haploid.
A diploid cell is represented by the symbol (2n) and is the chromosome complement found in any somatic cell--body cell.
In order to ensure the chromosomes of a sexually reproducing species remain constant from generation to generation, the sex cells must be haploid. By the reduction division accomplished by meiosis, a testis or ovary cell will give rise to haploid gametes. During fertilization, the two haploid nuclei of the gametes fuse to become a diploid zygote or fertilized egg. Therefore, the (2n) organism that is produced contains both a maternal and paternal set of chromosomes.
From this first diploid cell, the process of mitosis occurs resulting in an organism which has diploid body cells. During mitotic cell division, the chromosomes of the nucleus are replicated and the cell divides into two daughter cells after cytokinesis, which are genetically identical to the parent cell.
To answer the question then-- it is "better" for sex cells to be haploid and for body cells to be diploid in order for the chromosome number of the species to be maintained from generation to generation. The conditions refer to whether an organism is reproducing in which case it would require haploid gametes or merely growing, repairing or maintaining its body which would require diploid cells.
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