What is the wisdom behind some people saying, "Let us bury so-and-so next to his mother or father"? And is there any evidence that a righteous deceased person knows and feels comfort from those buried beside him?

The default is to bury each Muslim in a separate grave. It is not permissible to bury more than one person in a grave unless there is a necessity. A person should not be buried with their parents or others in one grave unless there is no separate grave available for them. As for burial in a separate grave next to relatives in the same cemetery, this is permissible and legislated, as the Prophet, peace and blessings be upon him, did with the grave of Uthman ibn Maz'un to bury those who died from his family next to him, and because it is easier to visit them.

As for a righteous deceased person knowing those next to them, or feeling comfort, or benefiting from the righteous, or being harmed by the people of torment, this is from the knowledge of the unseen for which there is no evidence to prove it from the Qur'an or Sunnah. The well-known hadith "Bury your dead among righteous people, for the deceased is harmed by a bad neighbor just as the living is harmed by a bad neighbor" is fabricated and false.