Temporary set top levels to TRUSTED

[Quest]

This feature seems to raise praises, but what does this mean? I mean what is temporary here?
I suppose it lasts until I close Firefox, but does revoking affect here.
And why this is so badly needed, and why it is on Options page when more fundamental settings like Default's is on that pulldown menu? (And temporay allow all too.)
And how dangerous it is runnig with this option on?