There are a few special units for different functions in Battle Spheres.
There are EXP Units that give a huge EXP bonus when fused to your units. Fusing units of the same element give an even larger bonus.
There are Coin Units that can be sold for a large amount of coins.
There are Spark Units that give a permanent stat boost when fused to your units. Different elements correspond to different stats; and depending on the rarity of the Spark, the boost can be stacked up to (rarity * 10) stacks overall.
This means that using 1 star rarity Sparks, a unit can receive up to 10 stacks of blessings overall.. i.e. (5 ATK and 5 HP blessings or any combination of stats) maximum. NOTE: This is 10 stacks of blessings OVERALL. NOT 10 stacks of blessing per stat!
After the first 10 blessings, units will require fusing 2 star rarity Sparks to gain up to 20 stacks of blessings and so on..