Why is proof beyond reasonable doubt needed for conviction in criminal cases?

The quantum of evidence required in the conviction of criminal cases is proof beyond reasonable doubt. This is defined in Section 2, Rule 133 of the Rules of Court, to wit:
In a criminal case, the accused is entitled to an acquittal, unless his guilt is shown beyond reasonable doubt. Proof beyond reasonable doubt does not mean such a degree of proof, excluding possibility of error, produces absolute certainly. Moral certainty only is required, or that degree of proof which produces conviction in an unprejudiced mind. (2a) [Emphasis ours]
The necessity for proof beyond reasonable doubt lies in the fact that "(i)n a criminal prosecution, the State is arrayed against the subject; it enters the contest with a prior inculpatory finding in its hands; with unlimited means of command with counsel usually of authority and capacity, who are regarded as public officers, and therefore as speaking semi-judicially, and with an attitude of tranquil majesty often in striking contrast to that of defendant engaged in a perturbed and distracting struggle for liberty if not for life. These inequalities of position, the law strives to meet by the rule that there is to be a conviction when there is a reasonable doubt of guilt.[1]

Its purpose is to balance the scales in what would otherwise be an uneven contest between the lone individual pitted against the People and all the resources at their command. Its inexorable mandate is that, for all the authority and influence of the prosecution, the accused must be acquitted and set free if his guilt cannot be proved beyond the whisper of a doubt. This is in consonance with the rule that conflicts in evidence must be resolved upon the theory of innocence rather than upon a theory of guilt when it is possible to do so.[2][3]

[1] Francisco, Evidence, 3rd Ed., p 576, citing State v. Postal Tel Cable Co. 53 Mont 104

[2] People v. De la Cruz, supra note 36 at 851.

[3] Salvador, Tranquil (2019). Criminal Procedure, page 251.