Structural Class Changes (Mana, Talents, Clear gish structure)

[Kryx]

The before usage is restricted otherwise spells with an attack like blight would always be cast with advantage on the attack roll.

If used before a spell is cast then the invisibility would immediately end - that's correct. The only reason I have the before wording is for the attack spells.

For normal usage it can be used for anything you use invisibility for. It provides the caster with a significant defensive ability against being targeted by attacks and spells. Avoiding opportunity attacks is a good example, or slipping past detection is another. Hide action could be used the next round if desired, though that's the same with the other invisibility spells.
You don't need to be behind full cover to benefit from invisibility: creatures still can't see you which means they have to determine the space you are in somehow and still attack with disadvantage. Many spells require sight of a target so those are foiled by this as well.