When it comes to splinterlands you need card assets in order to level up. At the moment these assets can only be rented or bought on the market and can not be filled via unopened packs (unless ordered on the secondary markets) Because of this rewards cards right now are one of the biggest boosts to new players to the game or those who only invested a little.
As a rough example just the other day I picked up an all new rewards card worth about $0.18 you can see my post about that here
It's a solid little card and in order to level it up I would need 5 total. This leveling up would give this card a +1 health which might not seem like a lot but just 1 health could be the turning point in some battles allowing me to rank higher and win some more battles.