To look impressive on a modern statue. It's basically a modern fantasy rendition rather than showing real martial arts technique. It is extremely unlikely that Sasaki Kojiro would have used such a grip in the duel with Musashi: he was noted for using an extra-long sword, and a grip like this provides no significant benefits while reducing the ability to parry/block effectively and reducing the effective reach of the sword. Why go to the trouble of having an extra-long sword and then reducing the advantage it gives? As the story goes, Musashi made a wooden sword that was even longer, and won due to the unexpected (by Kojiro) reach advantage it gave him. The pair of statues is accurate in that both Kojiro's sword and Musashi's wooden sword are extra-long:
There's a long traditional of inaccurate representation of famous Japanese fighters. For example, other depictions of the duel show Musashi with two wooden swords, contrary to the sources:
or even two steel swords:
This is just a projection of Musashi's use of two swords onto this famous duel. Another example is the conventional depiction of Tomoe Gozen with a naginata:
despite none of the early sources mentioning that weapon. These "errors" are deliberate, and serve a purpose. Wielding two swords makes Musashi recognisable to the viewer, since that's what he was famous for. Similarly, the naginata makes Tomoe Gozen recognisable as either a woman or a monk, and since she isn't dressed like a monk, and isn't Benkei, she's recognisable as a woman, and therefore probably Tomoe Gozen. An interesting variant is this depiction of Tomoe Gozen wielding a tree:
in which she is recognisable as Tomoe Gozen through being shown as a woman with superhuman strength (she was famed for being stronger than most samurai).
The statues of Kojiro and Musashi are recognisable by (a) sword vs wooden sword and (b) Kojiro's extra-long sword. The odd grip he uses isn't part of this; it's just a fanciful modern thing.
For some past discussion of such back-handed see
It mostly saw use (outside fiction) as emergency blocking/parrying immediately after the sword is drawn, and for ice-pick style stabbing. Neither of these would have been relevant to the Kojiro-Musashi duel.
