It is essential to do all of these things for two main reasons.
First, it is essential to do these things so that the investigation can continue to progress even after the crime scene has been cleaned up. If the evidence from the scene is not collected and preserved properly, it will be lost to investigators. They cannot reexamine it if they are unsure of something. If a new theory of the crime arises, they cannot check to see if the evidence bears it out.
Second, it is essential to do these things so that the prosecution can make a strong case in court. If the evidence is lost, this is not possible. If the evidence is not properly documented, it is also less possible to make a strong case. The prosecution will not be able to prove that the evidence came from certain parts of the crime scene or that it is really in the same condition as it was when it was collected
In short, all of these things are necessary to create a good case and to prosecute that case properly.