Singapore Airlines' Flight to SFO

I’ve been thinking about this for a long time, but thought some of you guys might be able to help me!

When Singapore Airlines’ Boeing 777-300ER is inbound for SFO, it flies via Hong Kong International. And outbound back to Singapore, it flies via Seoul. Any particular reason why Singapore Airlines does this? Is this for passenger connection or something?

SIA operates two daily flights to KSFO… flight 2/1 via VHHH and 16/15 via RKSI.

They make the stops because WSSS-KSFO is a very long flight that would have limited payload or interest (they only operate WSSS-KEWR and WSSS-KLAX) and they have 5th freedom traffic rights on all segments.