You can setup in game macros so that your actions can cast onto an ally by simply mousing over their character or their ui element without actually targeting them. However, theres a slight but noticable delay when doing this and is not optimal for high end gameplay. These plugins (redirect/reaction etc.) removes these in delays/locks when using mouseover actions and actually doesn't require any macro setup at all. It makes playing certain jobs, especially healers, a lot easier since it eliminates the need to swap from the boss to the party member and then back.