The Spell of Capturing is used to capture Boost Material Monsters. These appear in the middle of some stages.
I suggest that you evaluate two things when considering using one.
- Is the XP benefit great enough?
- Is the EP benefit worth it?
Sometimes it will be useful to capture the monster because they offer a huge XP boost for one of your monsters. I would put and the Prism Scarab in this category.
Other times you could use the hefty EP boost to help you on your way to getting a valued prize from the current event. The amount of EP you will get is correlated to the XP. For both of these numbers the higher up the level in the event that you are playing is, the higher these numbers will be.
In this picture you can see that you would get 2500 EP and 7500 XP (for Earth).