It works on autoattacks too, according to the description (though I haven't tested it).
However the primary reason to use it is to "load" a lot of damage into multiple single target spells and attacks on a champ who is going to benefit from more mana and MP5.
Corki and Ez, when played well, are not heavily reliant on their mana (excepting a few cases) or can be paired with a champ to reduce their mana reliance.
However champs like...
Yorick, possessing an AAR and a single target AD scaling nuke.
Pantheon, with a powerful AD scaling nuke and single target gap closer
Urgot, with a powerful and spammable AD scaling ability
Can utilize the entirety of the items uses, and are often encouraged to build some secondary mana sources (Trinity Force, Frozen Heart, Banshee's Veil) which amplifies the effectiveness of the item.