They are literally differing strains of the same plant. They are both Cannabis Sativa, Indica and Ruderalis. They are both capable of doing everything the other plant does. By selectively breeding you have more of some compounds than others - namely delta-9 THC. In fact, in the United States of America, Delta-9 THC is THE legal definition of 'THC' and the sole distinction between Federally illegal Marijuana and Federally legal Industrial Hemp.

This is a little more complicated (and some conjecture) but because of historical interaction and awareness of it. If delta-8, delta-10, THC-O and other compounds that intoxicate were as well distributed and known as Delta-9 THC, then the 2018 Farm Bill would have likely included all forms of THC. Who knows, it may have been an intended work around to allow the sales of intoxicating cannabinoids in states that do not have legal access to marijuana.

Industrial Hemp can and does produce delta-9 THC. When it is measured above 0.3% delta-9 THC (or thereabouts, as there is some leeway depending on the State), it is considered 'marijuana' or 'marijuana extract' regardless of whether the crop was considered industrial hemp or marijuana. If you purchase 'certified' Industrial Hemp seeds it is less likely to produce delta-9 THC because they have 'stabilized' phenotypes via selective breeding. However, this does not guarantee in any way that the crop would pass the 0.3% delta-9 THC benchmark. It is always a risk. When farmers fail this pass, they must incinerate their crop, losing their investment and time.

The distinction is arbitrary for an end use: regulation and enforcement. You cannot regulate until you define, whether through action or verbiage, so a definition was required.