The purpose of repotting is to :
Replace the old potting mix with new so that you remove any fertilizer salts,
Add new nutrients with the new soil,
And check the health of the roots of your plant.
Most plants will benefit from repotting about once a year. If you see a plant getting a long neck, or a plant that is too big for itís pot, then that plant should be repotted quickly. If a plant is getting root rot, then you should put in new soil that is better suited to the plant.