Is that the only way to know what fruit it will bear or are there other
reasons? How are new varieties created?

The varities we all know have a HUGE number of recessive genes. So
when you plant the seed, the fruit is not at all like the parent. By
grafting you get a genetically identical plant.

For breeding, seeds are planted. Even with careful crosses between
selected parents it take about 10,000 seedlings to select a new
variety.