I think the prosecution thinks the bleach is important because if the whole house was bleached down it explains them not finding Merritt's DNA at the scene.I think the prosecution’s dwelling upon this evidence creates speculation and hurts their case. As Furiously_Following, has pointed out, it’s not really conclusive of anything, and even if it was, it doesn’t implicate CM. IMO, they should move on to the financials which can be linked directly to CM, and is quite incriminating. Them spending so much time on something that’s not conclusive makes the prosecution’s case look like it’s nothing more than smoke and mirrors, grasping at straws because they really don’t have a case.
Same as the paint on the bra and the sledgehammer being tied to him painting over the walls.
It's a piece of the jigsaw to suggest the murders happened at the house and the crime scene was cleaned because they know they've got him at the house with the cheque printed.
Maybe they're worried that without a crime scene at the house the family could have been out and abducted by a carjacker while he was at the house only to steal a cheque. They don't want to risk the jury not being able to link the crime to the house. IMO.