In the episode of The Clone Wars titled "ARC Troopers," the Separatists attack the Kamino cloning facilities in hopes of destroying them and having Ventress steal the host DNA for the clone troopers. I understand the need to destroy the cloning facilities, but why would they need to take Jango Fett's DNA? Couldn't the Kaminoans just use DNA from one of the existing clones?
This is dealt with in the episode script;
- The Kaminoans can't make copies of a copy
"Ever since the unfortunate death of Jango Fett, we have had to stretch his DNA to produce more clones."
Clearly, having lost access to a primary source of his DNA, they're reliant on stocks of it. Stealing these would prevent them making more Jango clone troopers.
- Possessing his DNA is helpful scientifically to the Seperatists (not specified why, but I'm guessing bio-weapons or researching any physical vulnerabilities)
"Why not just destroy it?"
"Because the DNA could unlock new possibilities for us."