Sometimes, you might breed a Monster you already have.
Instead of selling the Monster for Gold, you could use the Extract section of Dr. Viktor’s lab to turn him/her into Cells.

The amount of Cells obtained from the Extractions, will be higher based on the level of the Monsters chosen. The Extraction process will consequently last longer based on the Rarity and the level of the Monster.

This way you are able to Rank up you higher levelled Monsters for future battles.