The first effect allows you to detach 1 Xyz Material from Castel to target 1 face-up monster on the field and change it to face-down Defense Position. That is Book of Moon as an effect on an Xyz Monster. 2000 ATK is plenty to take out many monsters in Defense Position, so what may have been a problem in Attack Position suddenly becomes far easier to destroy. This effect also cancels out the effects of face-up Monsters, which is always a big deal.
The second effect is even more powerful. If you remove 2 Xyz Materials from Castel, you can target a face-up card on the field (even Monsters in Pendulum Zones!) and shuffle it into the Deck.
Only one of these effects can be activated during a turn and only one effect of any Castel on the field can be activated in a turn.
Castel is a very powerful Xyz Monster. Being a Winged Beast also makes him a valid target to activate the popular trap card Icarus Attack, making him an auto-include in any deck that runs a lot of Winged Beasts such as Blackwings. However, both of his effects are powerful enough to see play in any deck. Castel sees play in the Extra Decks of many top decks. He has become an extremely valuable card, being reprinted in the 2015 Mega-Tins and as an ultimate rare in Astral Pack Seven, a set reserved for tournament prizes. He'll be one of the more played Xyz monsters for quite a long time.