Staining is sometimes necessary when viewing cells under a microscope because some structures of the cell may not be easily discernable. In such situations, the cells are stained to increase visibility of the cell structures. Also, staining the cells will create chemical bonds between proteins inside the cell and enhance the rigidity of the cell. This necessary for proper viewing under the microscope.
Staining is unnecessary in plant cells because plant possess cell wall which gives a rigid structure to the cell. Thus the rigidity of the plant cells does not need enhancement.