By dividing his power in half he can create clones of himself with half of his strength

Cell began quietly reflecting on his path to perfection. The decision to use the multi-form technique had been a stroke of genius...but not without significant risk. He had divided himself under cover of one of Trunks's attacks, his real self slipping into the water silently just moments before Vegeta's Final Flash had incinerated his clone.

It had been a bold move. With his power cut in half by the technique, he left himself open to attack if caught. That's why he had been forced to use the Solar Flare at the last second to stop Trunks and Vegeta. Moreover, merely absorbing Number 18's energy wouldn't have been enough to make up for the lost power; he had gambled on the transformation restoring him to full strength as well as adding 18's power to his own.

- Bringer of Death Chapter 54