I understand the word "import" meaning "to bring or carry in" from the Latin.
I have also read sources which say that the word "important" comes from "importare" meaning "being of consequence"
But how and why did the root word for "bring in" lead to a word meaning significant or being of consequence?
It seems the stem+prefix import already had the meaning of "care/attention" during the ages of Vulgar Latin. Compare Italian importare and French importer (to be important to, to have matter with). Both of them have totally lost the meaning of "bring in". However Italian importazione has the (original) meaning of "bring in".
There may be a change in Latin that caused the current state of mixed meaning (both "to carry/bring in" and "have significance"). Then it's a Latin question rather than an English question.
My understanding of the shift in meaning is that the usage of the word "import" meaning "to bring in" became synonymous with "of significance". And "importance" eventually became the more accepted form (perhaps because of differentiation.)
For example: in the Jane Austin Novel "Pride and Prejudice" a character is mentioned to have a question of "great doctrinal import." At the time of the writing of the novel (1813) the word "import" (aside from the bringing in of goods) meant "To bear or convey" or "To be of weight to" (see Webster's 1828 dictionary -- as the second and third definitions state.) So even at the time of the novel, the question could have been reworded as a question that "great doctrines bear" or "of bearing to great doctrines" or perhaps earlier usage would have been "brings in great doctrines" or "brings great doctrines into question."
SIDE NOTE: here's an interesting example of shift in usage of "import" to "importance" being used interchangeably in "The Catholic World" on page 744, published in 1887.
Now I'm not saying that the English usage is what changed the meaning but I'm using the examples and definitions above to show the similarity in usage and meaning. In 1800's upper English society, speech and thought across English, French, Latin, Spanish, etc., were very tightly knit, so it makes sense that the fluidity of the usage for the word "import" slowly lent it to move from "something brought in" to "something brought in with the implication of significance" to finally just "of significance."
This is one of the natural language changes that is hard to trace, but I hope to have illustrated the similarity in meaning enough to show how it could have naturally changed just by the usage. Whether it changed in the root language or later, the usage implying significance would have naturally changed the meaning over time.
I think if you look at the definition of "importance" as "value," you'll see the simple connection to trade: imported goods would the most valuable ones; those that would be worth the considerable investment of time and resources and still promise a profit:
https://www.thefreedictionary.com/important - "Something that is important is very significant, valuable, or necessary."
https://www.merriam-webster.com/dictionary/important Definition of important 1 : marked by or indicative of significant worth or consequence : valuable in content or relationship (First Known Use of important: 15th century, in the meaning defined at sense 1)
It is difficult for us in this day and age to imagine just how challenging transport over great distances in the 15th century and earlier could be. Exactly how the meaning shifted from commercial to general or philosophical significance, I think we would need Michael Quinion of "World Wide Words" to illuminate, but I can picture a 15th-century merchant threatening everyone with dire consequences over his expensively missing/delayed "import-ant" goods!
