Yep as said the respec scroll only refunds spent skill points so you can put them elsewhere, you can't change your class (at all currently without making a new character).
Benefit with these are that while initially levelling up you'll purchase skills and passives that you won't end up using as they're not suited for you or your build, therefore you can move them points to other skills, or even start crafting if you'd rather
I'd advise taking a look at Alcast or Gilliams builds to see what the 'Meta' is currently for your class (Mag DK) to see what you could allocate those points in to.
https://alcasthq.com/
http://deltiasgaming.com/
In case you didn’t know, the champion points you earn are shared by all characters on your account. If you’d like to try a sorcerer, create a new character. They can then spend the 144 champion points you’ve earned, even at level 3. They will be way stronger than your first character!