How do benign tumors affect the body?
It depends on the tumor location but mostly will cause local suffocation of nearby cells.
A tumor, in general, is a group of cells lost their ability to control their division properly. They just keep dividing and dividing chaotically in an aggressive manner without any regard to nearby cells.
Usually in normal cells, division is induced by the need of more cells of the same type and inhibited by achieving that goal. All cells are equipped with a mechanism called "contact inhibition" and "cellular aging".
1) Contact inhibition is the immediate suspension of cellular division when the cell comes in contact with other cells. This serves to prevent cells from overgrowing their designated tissue structure and not to "suffocate" other cells by growing over them.
2) Cellular aging is the fixed rate at which a cell can divide. Almost all cells have this programmed in them. They will keep dividing until they hit a certain number of divisions and then stop dividing.
Most of the time, tumors or cancer cells lose those two mechanisms, resulting in continues uncontrolled growth that will certainly cause damage to the nearby tissue(s).
Malignant tumors can reach the blood stream and a few cells would stream down the blood only to land in another organ and start overgrowing again. This is called "metastasis" and its the reason of cancer's fatality.
Benign tumors can't reach the blood stream so they just overgrow in their spot. The severity of the tumor varies depending on the location. A benign tumor in the brain is life-threatening, while a benign tumor on the skin of your forearm is as harmless as a mole on your skin.