I'll take a stab. I'm still learning about his stuff myself, so anyone please feel free to correct anything I get wrong. I look for SP rated because I want to ensure I get oils that have the correct additive package/chemical composition for our cars. As I understand it, oils that have a high SAPS content, or high ZDDP content can be damaging to the catalytic converters. We have two GDI, cars, one of which is a Turbo GDI. As I also understand it, if an oil has too much calcium, it can pre-ignite in the combustion chambers, leading to catastrophic damage, particularly in the Turbo car. For those reasons, I follow the recommendation in the owners manuals as to what spec/certs to look for in the oil. The Turbo car is a little older. SN was probably the standard when it came out, but it's also a European car, so I go off the European standard when looking for oil for that car, specifically A3/B3 or A3/B4.
With regard to API being only an American standard, it is also listed in Japanese, Australian, and European owners manuals. I believe the intent is that if you are, for example, in Belgium and drive a car that calls for an oil that meets the ACEA A3/B4 standard, but you can't find one, the owners manual will also state an API spec that meets that same standard so that you have more options in finding the right oil for your car.