Everyone loves the disadvantage of having to use 60 cards
Assuming you have two (Id reckon youd have two Megas if youre using Calem in your deck), and your opponent uses one, you get a good case of drawing 3 cards
If your collection is full of older cards, then there is some serious potential for money as older sets have a variety of valuable cards and some sets are valuable simply due to their age
This is mostly to avoid the look of impropriety, as being able to see the back of the card means theres the possibility of marking (i.e., putting marks on certain cards so you can tell when they pop up in your deck without drawing/scrying them)
for the rest of this turn, that monster you control gains 1500 ATK/DEF, is unaffected by your opponent's monster effects, and the next time it would be destroyed by battle, it is not destroyed
Each pack contains an assortment of 15 Pokmon cards sourced from various sets and years