When is it useful?

I'm no expert, but since it forces your Pokemon to attack last, it could allow you to synergise moves which require you to be on lower hp.

I'm sure that there is some kind of scenario in which it is a good item ;)

151455 When is it useful?

Often it good for tanks or some pokemon using some special skill, for example Payback (double attack if it do last) or Counter, etc

PRobably when your poekmon has analytic or something like that. Analytic increases the item of the attack if used second. Does it work though?