I don’t use Roundcube, but from the looks of it the NC app is a somewhat narrower integration than that provided by Snappy or NC Mail. Namely it enables somewhat more friction free usage of Roundcube alongside NC without tightly integrating them (beyond optional authentication). If that fits one’s primary requirements, great.
That said, it’s not silly - it’s just a different use case. And - from your reaction - perhaps your use case simply isn’t the same. No biggie. 