There are six disciplines taught at the Seminary of Magic on Androva, and one of them is Remedies. Every underage magician learns how to make a Portal Remedy first. You can't complete the year one assessment otherwise.
The thing about portal travel is that in the beginning it's exhausting. You might just be walking through a doorway, but it's a doorway constructed from magical energy. You could be travelling miles and miles in a split second. Or you could be stepping into another world.
This remedy obviously helps Shannon cope with travelling to Androva in Stealing Magic. But if you have read Capturing Magic, you will know that the humble Portal Remedy has an important role to play in that adventure too!