Let's talk about two things that used to not go together: Japan and butter. Like other East Asian cultures, butter was never a part of the traditional Japanese diet, and was actually treated with disgust when introduced by Europeans in the 19th century.
(Fun politically-incorrect fact: When living in Japan, I learned that the word "butter" was used in an outdated anti-foreigner slur. Both Westerners and overtly Western things were referred to as bata-kusai, "kusai" being Japanese for "stink." It was thought that eating butter produced uniquely European body odor, hence the slur was "butter stinkers.")
Today Japan has accepted butter (particularly where baked goods and confectionaries are concerned). Uptake isn't as brisk as in America or butter-crazy France, but it's produced locally (in the Hokkaido region) and consumed in enough quantities that the country experiences occasional butter shortages, like this one in 2014.
Also, butter in Japan doesn't come like butter in the 'States: It comes in slabs, as they've adopted the traditional European form factor. (I believe it's just us Yanks that form butter into sticks.)
My speculation--and this is based purely on the year I spent living there, during which time I witnessed fantastically anal-retentive table manners--is that a) This is for those who don't want to grate the surface of their butter, which probably gets messy as you work your way down through the slab, and b) this satisfies the Japanese need for order.
In other words, for us Americans who want a pat of butter, we just cut one from the stick; but for Japanese users faced with a slab, a crosswise slice would be too unwieldy to balance on your average butter knife.
An alternative would be to cut more manageable diagonal slices--i.e. cut a corner off of the slab--but I'm guessing a slab of butter with 45-degree angles cut into it would be too visually chaotic for Japanese sensibilities. This "tool" leaves behind an orderly 90-degree cut.