Here's why:

* Storage of blood cells: The spleen holds a reserve of red blood cells, white blood cells, and platelets. This allows the body to quickly release these cells into circulation in case of blood loss or infection.

* Filtering of blood: The spleen also filters old and damaged red blood cells from the bloodstream.

* Immune function: The spleen contains a lot of white blood cells, which play a crucial role in fighting infections.

However, it's important to note that the spleen is not the only organ involved in blood storage and regulation. The liver and bone marrow also play vital roles in these processes.