I think it also emphasize the fact that the pieces sent were of great quality.
It means, whoever will read these letters (the expert or you maybe), will have no choice but to reach the conclusion that these pieces content (la teneur) is impressive.
If you say ... dont on pourra apprécier la teneur, which is very close, there is still a possibility that these things are good but not as valuable as the family claims. It's a bit subtle.
Is "chacun" like "tout le monde/n'importe qui" here ["whose contents anyone can appreciate/can be appeciated by anyone"?] Note that in English, the verb "appreciate" can mean either "be thankful for" or "understand", presumably the latter here.