Because a tree knot hole normally is the diseased part. Most trees would be able to fight the disease, and the knot hole would be a helpless lump, but in some cases, filling the hole with cement can stop the disease from traveling. If you are talking about To Kill A Mockingbird, Mr. Nathan did it to stop Boo from communicating with the outside world
Tom Robinson plugged the knot hole with cement in "To Kill a Mockingbird" to prevent Boo Radley from leaving his house and interacting with the children, as the adults in the neighborhood felt he was dangerous and should remain isolated.
He said the tree was old and it was dying and that people fill trees with cement when they're sick.
Nathan Radley tells Jem and Scout that he filled the knot-hole with cement because the tree was dying and he didn't want it to fall on someone. However, it is later revealed that he filled it to prevent gifts being left for the children by Boo Radley.
Nathan Radley puts cement in the knot hole to stop Boo Radley from leaving gifts for Jem and Scout. He believes that Boo's interactions with the children are inappropriate and wants to cut off their communication.
The knot hole in the tree likely rotted away over time due to exposure to the elements, insects, and decay processes. As the wood around it deteriorated, the knot hole eventually disappeared or collapsed, leaving a void in the tree.
Mr. Radley filled the knot hole with cement to prevent Boo Radley from communicating with the children. He believed it was for their own safety and to keep Boo isolated from the neighborhood.
Jem and Scout discovered that someone had filled the knot hole with cement, sealing off the opening where gifts had previously been left for them by Boo Radley. This action by Nathan Radley signified the end of their secret communication with Boo.
Mr. Radley cements the knot hole shut.
The cemented hole in the tree represents Boo Radley's isolation from society. Nathan Radley fills the hole with cement to prevent Boo from communicating with Jem and Scout. Jem is heartbroken because he realizes the cruelty of this action.
Nathan Radley knows Boo has been leaving gifts there for the children so fills the hole in with cement. When the children enquire as to why he's doing this, he lies and says that the tree was dying and this would save it.
As the tree grows, the knot hole can become covered up by new growth or get pushed out over time. The wood around the knot hole may also continue to grow and expand, filling in the space where the knot hole used to be.
A hole in a tree, sometimes animals live in them