Theres lots of literature regarding the BOJ's obscenely ultra
low interest rate widely available on the net and I won't dwell
with its technicalities and historical evaluation to bore you to
death.
In a nutshell:
What do you think is the main business of the Japanese? Theirs is a
predominantly export driven economy and Japan exports a huge
fraction of the world's supply of foreign goods to the rest of the
world. Think in terms of your SONY, PANASONIC, TOYOTA, HONDA etc
etc. Therefore, in order to propel your main source of business,
you have to keep your costs low, otherwise foreign investors are
going to find other sources of cheaper alternatives (think
China).
A manipulation of the BOJ's interest rate to an obscenely low rate
keeps the costs of borrowing the Yen cheap, encouraging foreign
investment and keeping export costs low, not forgetting various
political agendas has influenced the rate to reach a near 0%
level.
Problems with these cheap and seemingly never-ending supply of Yen
from Japan? You bet. Foreign investors borrow the Yen, at the low
low rate provided, and invest it into higher yielding equities
around the world for larger returns and fatter margins and their
borrowing at the rate of billions. Most of the assets and equity
prices that we see nowadays are therefore driven up and speculated
on by these infamous "Yen carry trades". A 0.50% point hike by the
BOJ is enough for a mega institutional investor to rack up huge
drops in profit whom leverages on these Yen carry trades ( Loss of
profit = 1,000,000,000 X 0.5 ). As many factors (political,
domestic, macroeconomical etc) can undermine the BOJ's decision to
hike the interest rates, economist predict that the "unwinding" of
these huge Yen carry trades will be the catalyst of a major
financial meltdown on a global scale.