Just for arguements sake, I'll break assist methods down to two categories.

Type A:
#show
/cast [target=focustarget] Lightning Bolt

#showtooltip
/cast [target=party1target] Lightning Bolt

#show
/cast [target=focus] Lightning Bolt

#show
/cast [target=targettarget] Lightning Bolt

#show
/targetlastenemy
/cast [target=party1target] Lightning Bolt
/cleartarget



Type B:
#show
/assist [target=focus]
/cast Lightning Bolt

#show
/assist [target=party1]
/cast Lightning Bolt

#showtooltip
/tar [target=focus]
/cast Lightning Bolt

#show
/assist
/cast Lightning Bolt

#show
/targetlastenemy
/assist [target=party1]
/cast Lightning Bolt
/cleartarget


Not a whole lot of difference between any of these. Each uses the assist method of your choice.
In the type A examples, your slaves cast at the target of your choice without ever selecting that target.
In the type B examples, your slaves are targeting whatever you have chosen them to cast at.

Type A is generally very useful in PvE to ensure before a pull they're on the right target.
Type B is generally better for PvP, as the opposition has no clue what you're going for next.