Just for anyone who might need clarification: this doesn't mean they're the most valuable company in Japan, they're far from it (around #58 based on this ranking by Forbes https://www.forbes.com/global2000/#b039e1335d86). This just means they hold the most "net cash", which is calculated from their total cash and deposits, minus debt and bonds.
I've always wondered why Nintendo holds this large of a "war chest". It's probably related to the fact that they're so focused on a single business unit (game hardware and software) and if that market falls, they can sustain themselves for multiple generations of "failures"... Which might explain why they're diversifying their business so much in recent years, at the back of their strongest asset, which is their IP.
I wouldn't be surprised if, once they feel comfortable in other markets, they start spending all of this cash on some acquisitions. With that amount of money, they could buy massive japanese or international studios. They could buy the remaining 85% of DeNa multiple times over, or they could buy all of Bethesda's parent company (ZeniMax Media) at least twice. They could even buy almost the entirety of Ubisoft. Not to mention other smaller devs or publishers. They could buy an animation studio and produce their own films (if Pixar is valued t $2B, imagine what they could do with $8B).
Anyways. It's a lot of money, and it's odd that it's sitting around so idly.