Why is chromosome analysis recommended for gender test of a child?

That is just a good starting point. If you don't know what sex someone was supposed to be, then karyotyping can be a good place to start. It can also give a clue as to what underlying conditions there might be. For instance XY chromosomes and natural feminine features could indicate androgen insensitivity syndrome. On the other hand, androgynous features and an extra sex chromosome would indicate Kleinfelter's syndrome.