Question

What is transplantation? Why it is preferred for some plants?

Answer 1

Transplantation is a technique to move a plant from one location to another.
Roots grow outward, so they naturally collect at the edges of the container and leave much of the soil in the middle of the pot unused. This will cause slowed growth because the roots may restrict each other. 
eg: tomato

Answer 2

Transplantation in plants is the process by which a part of the plant is cut off from parent plant and fixed in the soil for the growth of another plant. It is necessary because if the mother plant is infected the branch or part of bud can be cut off and fixed on another to allow their growth.