What are these used for? Do they have any effect on how the empties are used, or are they just to help the user identify them, etc.?
Do they have any effect on how the empties are used
Use the shape and size that you prefer, except for the Image option. If you need to know what each axis is then select one that shows the each axis. If you need something that indicates some sort of direction use one with an arrow? If just a point in space maybe a cube/sphere which may be easier to see in your scene.
I don’t really see much use for these other than maybe the image option. Otherwise they have no additional functions. It’s just the user’s preference.
They affect the use of empties, if that use ever involves looking at the empties. I find that to often be the case.
To add another example, you can have several empties on the same location while keeping them distinguishable and selectable.