Scrolls are 1 time use items, with the exception of Ivory Scrolls.
Lets say I have a Necromancer, who knows the spell Raise Zombie, and I attain a scroll of Raise Zombie. If I cast Raise Zombie, and should fail, I can instead expend the Scroll to successfully cast the spell. You could hypothetically just use the scroll right away, but it is redundant to not at least roll to see if you successfully cast the spell in the first place.
If I have an Enchanter that does NOT know the spell Raise Zombie, then I may expend the scroll, to successfully cast Raise Zombie, however I of course do not get to roll beforehand, to see if I would successfully cast the spell.
Scrolls are just 1 time use items that let the user (assuming they are a spellcaster) successfully cast a spell indefinitely.
If you have further questions on rules, there is a thread at the top of this page that is pinned, and Joe (the creator of the game) often answers questions on the rules.
Thank you,
Like Clockwork