When deleting a node from a binary search tree that has two children, the reason we can

replace its element with the greatest value in its left subtree is because

the value will be greater than everything on its left, and since it was in the left subtree, it will be less

than everything in the right subtree

the value will become greater or equal to the values of its children

Greatest value is the easiest to move

We don't replace nodes when removing from a BST