It is necessary because without a secure chain of evidence the prosecution cannot prove that it was not tampered with or contaminated by some slob of a cop that may just not give a crap.
They are not similar but NOT the same. Preponderance of the evidence is the standard for juries to follow in CIVIL trials. Reasonable doubt is the standard for guilt in CRIMINAL trials. NOTE: It is...
Preponderance of the evidence means whether it is more likely or not. Beyond a reasonable doubt means that there is no other explanation than the accused is guilty.