Focus Macros are for using some abilities without having to lose the target you are already nuking.
For example in 2v2, healer and dps, using the focus macro will make the fight easier and faster as the cc abilities will be casted without losing time when switching targets. It will save 2-4 seconds depending how fast the player is.
An example of focus macro for hunters:
#showtooltip Scatter Shot
/StopCasting
/StopCasting
/StopCasting
/cast [target=focus] Scatter Shot
/castsequence reset=3 Trap Launcher
It works the same for every class, for polymorph, fear, blind, etc.
A polymorph focus macro for a mage would be:
#showtooltip Polymorph
/StopCasting
/StopCasting
/StopCasting
/cast [target=focus] Polymorph
How to target a player in arena?
Just by doing right click on his character icon.
This can have even use in PVE if the fight requires lots of fast switching, like tranquilising shot on a different mob (hunters).