There has been a list of Dr.Strange's spells from the Vishanti book for 40 years. A plot device implies that the powers are new to fit the situation, but for Strange, it isn't so. He always uses the same spells from the same book that has been around for 30 years. The only time he used something new was during his black magic stage.
So, blammy! Dr.Strange isn't a plot devicer. 😠