I suspect
@Barque of Dante is communicating he agrees with your statement
"I don't see why
The preamble is just a means of introduction."
xxxxxxxxxxxxxxxxx
I find that agreements between people (as in marriage) or mission statements by corporations or institutions typically attempt to express or highlight certain values.
Yet, few people attempt to extract from the Constitution even a HINT of what values were important at the Constitutional Convention.
The Preamble does that and should be considered by those tasked with interpreting the Constitution (SCOTUS, primarily). This approach is a more nuanced and wholistic view of the Constitution than than trying to divine the everyday common behaviors and attitudes of 18th American colonialists as a standard for jurisprudence that is labelled "originalism".