That's a half truth.
It has also been said that the purpose of the law of evidence is to promote the search for the truth about facts.
The truth-value of this proposition is much less -- perhaps, say, 1/20th.
The American legal realists (including John Henry Wigmore) recognized (and argued) that all branches of law (including the law of evidence) serve a variety of interests and purposes. Hence, evidence scholars with a "realist" cast of mind acknowledged and argued that the law of evidence is shaped by a variety of factors other than the desire to determine the truth or falsity of propositions about legally-material factual hypotheses.
But the characterization of evidence law's non-epistemic purposes as "interests" stultifies thought and analysis.
The "interests" that shape the law of evidence may have their own logic or structure. One may have to understand the structure of each such interest or purpose if one is to understand the workings of the law of evidence in a given society at a particular point in history.
It is true that the law's purposes -- including the purposes of the law of evidence -- cannot be pulled out of heaven; different societies have different characteristics, interests, and purposes. But these historically-contingent purposes, or "interests" -- which in many or most societies do include a yearning for "truth" -- do have a structure. For example, "confrontation" (in adversary legal proceedings) may be an "interest" but it very probably has a certain connotation in the minds (and hearts) of some people in a society such as ours, and it may be possible to get a sense of what such an "interest," or value, means. Similarly, with notions such as "closure," "dignity," and the like.
A coherent effort to construct a systematic exposition of historically-contingent proof regimes should probably attempt to (i) identify the congeries of "interests," purposes, and values (and also non-historically-contingent constraints or variables) that shape or might the law of evidence at a particular point in space and time, (ii) put those interests, values, and variables into something resembling a matrix, and (iii) then -- to mix mathematical analogies -- describe how such interests, etc., interact to produce the vector that is the law of evidence in a particular society at a given point in history.
It would be extraordinarily difficult, of course, to provide such a systematic description of the law of evidence. But such a description is the one to which systematic scholarship ("theoretical scholarship") about the law of evidence should probably aspire.
coming soon: the law of evidence on Spindle Law