Macros aren't very good for having one skill follow another. Their timing is inefficient, and they're too subject to being broken by lag or other interruptions for that to be a good idea. (The timing inefficiency was deliberately placed there by SE to ensure that players who play directly by manually activating each skill have the advantage over those who overuse macros.)

They can, however, be useful for things like situational targeting, where you'll have a skill apply to either your current target, your target's target, or your focus target, depending on who/what you have targeted at the time. They're also sometimes used to combine a party notification with a skill, like (as mentioned before) when a healer announces that she's casting Raise on someone so that the other healer in the party doesn't waste a Swiftcast on trying to Raise the same player. Those are the sorts of things they're most useful for.