why we disable writing in color and depth buffer before stencil test and enable it after stencil test in reflection?

void display (void) {

     glClearStencil(0); //clear the stencil buffer
     glClearDepth(1.0f);
     glClearColor (1.0,1.0,1.0,1);
     glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT |GL_STENCIL_BUFFER_BIT);
     glLoadIdentity();
     glTranslatef(0, 0, -10);
     //start
     glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE); //disable the color mask
     glDepthMask(GL_FALSE); //disable the depth mask

     glEnable(GL_STENCIL_TEST); //enable the stencil testing


     glStencilFunc(GL_ALWAYS, 1, 0xFFFFFFFF);
     glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE); //setthe stencil buffer to replace our next lot of data

     floor(); //set the data plane to be replaced

     glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE); //enablethe color mask
     glDepthMask(GL_TRUE); //enable the depth mask

     glStencilFunc(GL_EQUAL, 1, 0xFFFFFFFF);
     glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP); //set the stencilbuffer to keep our next lot of data

     glDisable(GL_DEPTH_TEST); //disable depth testing of thereflection
     glPushMatrix();
     glScalef(1.0f, -1.0f, 1.0f); //flip the reflection vertically

     glTranslatef(0,2,0); //translate the reflection onto the drawing plane
     glRotatef(angle,0,1,0); //rotate the reflection
     square(); //draw the square as our reflection
     glPopMatrix();
     glEnable(GL_DEPTH_TEST); //enable the depth testing
     glDisable(GL_STENCIL_TEST); //disable the stencil testing

    //end

    glEnable(GL_BLEND); //enable alpha blending
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); //set the blending function

     floor(); //draw our bench
     glDisable(GL_BLEND); //disable alpha blending

     glRotatef(angle,0,1,0); //rotate our square
     square(); //draw our square

    glutSwapBuffers();
    angle++;
 }