But I also understand that for internal High Sierra booting SSD, it is better to format as APFS, and that is why Apple does that automatically without allowing users to do otherwise. Right?
Better or worse doesn't matter. You don't have a choice. The HS installer will convert all-flash storage-equipped Macs to APFS with the emphasis on all-flash. As I wrote don't convert it on your own first, because I don't know what will happen if you do. Go ahead and convert it and find out for yourself. Then, you can be the expert.
All I know is that there have been many reports of problems from those who did, but their underlying causes are many.
But if that is so, my question is: what is the advantage of APFS for internal High Sierra booting SSD that do not have external ones?
APFS has certain undeniable advantages that you can read about for yourself here:
https://developer.apple.com/library/content/documentation/FileManagement/Concept ual/APFS_Guide/Introduction/Introduction….
That's all I can tell you, and the only thing to add to that is that Apple's concern is primarily with devices incorporating built-in all-flash storage. External devices are way, way down their priority list.