having 4 packages means you have a 1/27 (I lost count of the total of scorestreaks) chance of getting what you want per package, which sounds like reasonable odds for your average gacha gamer. It just so happens that the gacha gods decided that I need 3 Shock RCs against bots in a ranked match, hehe. That's kinda where that reroll perk comes in, you get to reset the package once to get something more in tune to what you wanted.
It can actually screw you up in a proper ranked match though, so unless you'd want to go gambling, you're better off using other scorestreaks instead.