The organ that humans do not need is the Appendix.
Humans can easily live without an Appendix and a lot of people have the Appendix removed before it bursts and live just fine without it.
Other organs and body parts that you can live without are wisdom teeth, teeth, Coccyx, External Ear, Male Nipples, Arrector Pili, Plica Semilunaris.
The only body part that cannot repair itself is the teeth.
While new teeth can grow once baby teeth fall out the adult teeth cannot regrow or repair themselves once damaged and need dental work to repair and treat.
The only body part that grows back is the skin which can regenerate although the liver is also an organ and body part that can also grow back.
Other parts of the body that can grow back are fat, brain tissue, fingernails, toenails,
Regeneration means the regrowth of a damaged or missing organ part from the remaining tissue.
As adults, humans can regenerate some organs, such as the liver.
If part of the liver is lost by disease or injury, the liver grows back to its original size, though not its original shape.
Although some people who have a diseased portion of their liver removed are unable to regrow the tissue and end up needing a transplant.
Researchers from Michigan State University believe blood clotting factor fibrinogen may be responsible.
While some brain tissue can sometimes regenerate the brain actually can't regenerate itself well because when the brain is damaged its cells find it harder to make new ones.
This is because the brain has very few of the special cells, or stem cells.
Humans can't regenerate body parts other than skin and tissue or nails because mammals have more complex biological structures; limb regeneration would require sophisticated controls to ensure that limbs and organs don't grow out of control.
Humans, for example, are already equipped with safety mechanisms to ensure that individual cells don't grow uncontrollably.