The difference between growing plants from seed and propagating plants from cuttings off a parent plant is genetic variation.
The seeds of many fruit produce trees that are different from the parent, because seeds themselves are produced by sexual reproduction – they receive genes from a male and female to form. As they are a cross from two sets of genes, many fruit trees are not “true to seed”, that is, their seeds will produce a different variety of tree from the parent. For the botany purists, yes, there are some exceptions, but this is generally the case.
Propagation methods that use material from the parent trees such as cuttings are a form of vegetative, or asexual reproduction, as genes only come from one parent to produce identical genetic clones.
Let’s have a look at a real life example to better understand this concept. Imagine we want to produce more apple trees, say Granny Smith apples from an existing tree. Apples are not ‘true to seed’, so the seeds from any particular variety apple will not grow to be the same variety as the apple tree they came from. In our case the Granny Smith apple seeds will produce a wide variety of different and unknown apple tree types.