SI has four main advantages over the American system:
The most-often cited, but least important, is that different units for the same quantity in metric have conversion factors that are powers of 10. This is convenient, sure, but hardly necessary.
The second-least important benefit is that it’s the system used by the majority of the world. Things work better when everyone uses the same units, and it would be easier for us to change to match all of them than for them to change to match all of us. Though admittedly, this isn’t anything inherent to SI; it’s a quirk of history (albeit one driven by the other reasons).
The third benefit of SI is that it’s what’s called a coherent system. The base units for various quantities are derived by multiplying or dividing base units for other quantities. If you take the SI unit of force times the SI unit of distance and divide by the SI unit of time, you’ll get an answer in the SI unit of power. Tell me, how many foot-pounds per second are there in a horsepower? I don’t know, but car people need to know, and they shouldn’t need to know.
The fourth benefit is that it’s a standard. You can’t compare the meter to the foot, or the liter to the pint, because there is no the foot, or the pint. Distance measurements can be statute or surveyor’s (or, in the case of the mile, nautical). Volume units can be American or Imperial. Weights can be Troy or Avoirdupois. I’ve seen old rules that had twelve different inches marked on them, for different countries (thankfully, that’s nearly obsolete now, because most of those countries have abandoned their inches in favor of metric). And I have a recipe that I can’t make right, because I got it in Ireland, and I don’t know if the units are imperial, for a British Isles audience, American, for the tourist trade, or a mix, because the author tried for one or the other but knew about some but not all of the differences. If the recipe had instead been in liters and grams, I’d have no such problem, because a liter is always a liter and a gram is always a gram, for everyone.