Here's one: why would someone outside of Japan ever want to loan the Japanese government money at ~0% interest?
cfr.org/blog/puzzle-ja…
They're only loaning the Japanese government money (via buying JGBs) as a side effect of the transaction that they *really* want to do, which is selling dollars [0] to Japanese financial institutions, which is lucrative.
[0] Technically, dollar-denominated exposure.