So, in Japan, this has a couple of functions, but one major one. By keeping salaries low and offering bonuses, employees can basically be only compensated the bare minimum in the case they (a) are no longer wanted (since firing is very hard here), (b) not performing as well as expected for whatever reason, or © the company did particularly poorly.

As mentioned, it ties into one of the levers they have to pull for under-performing or bad-fit employees they might want to get rid of in a country where workers have a fair bit of rights on that front.

On the other, it does make some applications/calculations a little weird as some home loans etc. have repayments that expect those bonus payments (either a higher amount twice-yearly or two extra payments per year). Most companies in Japan pay monthly (and most of those on the 25th or closest preceding business day).