"Given that either way, you would have to be ready, it doesn't make a huge amount of difference. However, there is an advantage. If you're going into a situation that's questionable? You can have them already on you. Secondly, there are times you don't want to give away that you've picked up on someone coming up to you. Hand in your pocket, silent cast, means you can survive a hit they'll throw. Here, this is a particularly good one..."

A mirroring spell. This one turned into a sort of magical mirror that was more limited in size, couldn't cover the whole body, but it could reflect a spell back at the attacker.