This could be helpful in optimizing your lead database.
Let's say you have a smart list of Bad Fits that run off of @domains in the Lead Database.
And a smart campaign that marks everyone in this smart list as Marketing Suspended.
It works well when a new lead is created, but what if you add a domain to the smartlist? You have to manually run the campaign, then add the triggers back in. It would be ideal (and efficient) to have a trigger that says Added to Smartlist. (and one that is Removed from Smartlist).