It has beneficial modes for you and detrimental effects for your opponents, it plays to different aspects of the game, messing with the battlefield, graveyards, and life totals, and it does all of this at instant speed
"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack." You must still follow any timing restrictions and permissions, including those based on the card's type
More on this exception below
If you buy some of these and get caught by a reprinting, buy new copies until your average price paid is satisfactory to you than watch all of them grow
Phasing out an opposing creature is a good way to get a blocker out of the way, but because you can put the Gate into play from your hand you can Phase things out at instant speed too