Cell division means reproduction, and in multicellular organisms it means growth. Cell division is important to replicate new DNA so that you don't die.
In the first cross one of the parent has only dominant allele and one has only recessive allele that means the offspring in F1 will be 100% heterozygous
XxYyZz
the gametes resulted from this individual are
XYZ XYz XyZ Xyz xYZ xYz xyZ xyz
it produces 8 types of gamets one of which is xyz the proportion is 1/8
