Assign button for Materials

I use the Assign button for using different Materials on specific faces. Or if a Material doesn’t get applied to an object when I want it to. However, I am unclear on the specifics of when to use it. I also never use Select or Deselect.
Can someone clarify for me? Thanks!

Multiple Materials

Assign = assign the selected material to the selected mesh faces
Select = select all the faces that use this material
Deselect = deselect all the faces that use the selected material

Use these tools when you want to do these tasks

Note that the first material you add get automatically assigned to all the faces of the object

Wow that was so simple. Thanks Richard!!!

Is it common practice to switch to Edit mode, just to have access to these buttons?
Occasionally I cannot seem to apply a Material to a mesh while in Object mode, so I resort to using the “Assign” button in Edit mode.