If this country built a strong cannabis industry, right now, what would the benefits be?
Cannabis in the Industrial Hemp strain does not produce enough THC to get you high. The THC-production-ready strains represent an emerging market thanks to recent changes in the legislative climate.
All cannabis will grow in less-ideal conditions. It is easily grown and harvested on land unfit for the growth of more sensitive crops. See discussions about switchgrass for this concept; the benefits here are the same. Better land utilization means increases in economic wealth, as land is a wealth-bearing asset which holds less value when unused (You own land, but produce nothing? You can SELL it, but you're not gaining wealth by PRODUCING on it).
Industrial Hemp provides strong fibers which you can blend with clothes in 30% hemp 70% cotton to make cotton-like fabric of extremely light weight--sort of like silk--with high durability. Higher hemp content would be perfect for labor-clothes (i.e. denim), as it's ridiculously hard to cut and tear. Hemp is also very smooth and so very comfortable. Spun hemp fiber, being that hemp has better tensile strength, doesn't break down as easily under the stresses of wash and wear, and so produces less lint, so the clothing lasts longer.
Longer lasting, higher-quality clothing made from lower-cost materials means poor people can purchase clothing at a discount price and less often. They are then able to more effectively manage their money, improving their economic situation. The middle class and the rich similarly benefit, ending up with more money to spend elsewhere and stimulate other economic sectors. This represents an increase in wealth via a decrease in the destructive turn-over of goods (i.e. things don't wear out as fast, so are not destroyed as often; and the lower resource intensiveness of production reduces the amount of wealth sunk into creating the good, thus greatly increasing the wealth of society by replacing a high-cost good with a low-cost good of equal or greater value).
Hemp seed is highly nutritious and can be used for feed or food. Hemp seed oil can be processed into biodiesel fuel. Again, this allows for the use of unsuitable land toward a valuable economic end, thus increasing the wealth of society.
Hemp damages the cotton industry. The cotton industry, being large and powerful in the time of slave-negros, thus lobbied congress as is American tradition to produce protectionist laws. Hemp was, for a time, heavily regulated; moving onto hemp production would today require some relatively large start-up costs, despite that the process of spinning plant fibers into thread and yarn is largely the same. It would also incur a frightening amount of risk due to the risk of accidentally growing recreational cannabis (the plant is the same, the seeds look the same, and pollen on bees and in the wind from nearby grow operations could taint your crop and produce high-THC seeds). Government regulation of recreational cannabis would require regular DEA inspections, meaning expensive permits to cover the cost of these inspections, as well as the risk of determining that cross-pollination has created a hybrid and your entire crop can give folks a weak buzz--so you must now raze and burn it at your own expense, a huge loss of wealth.
We would have been better off if we didn't ban the stuff. Jefferson was, in a way, right. Maybe a little overzealous--it's a great crop with wonderful uses and a huge amount of economic benefit, but it's not the absolute top-priority of anything--but he was right.