It shows the tooltip for the skill.

I.E. On my warlock, I use this macro to help me condense spell, so they take up less space on my bars.

#showtooltip Fel Armor
/cast [button:1] Detect Invisibility
/cast [button:2] Undieing breath
/cast [button:3] Fel Armor

What this does is the spell icon, and the tool tip (that bit of info you see when you mouse over a icon) for fel armor. When I left click it, it casts detect invis. Right click casts undieing breath, and middle mouse click casts fel armor.

It also helps with cast sequence macros, which would change icons as you use them, and also not show any tool tip at all when you mouse over them. You can make it always show the first icon.