Basically they believe their stock price is undervalued so they will do a buyback and offer the stock again at a later time when the prices have gone back up. At least that's how I understand it.
A likely scenario.
It could also be a case where they don't own enough of their own company, and want to buy back some stock to prevent a takeover...
No, a buyback doesn't change anything in that regard. It doesn't give Ninty power as a shareholder. It just leaves fewer shares outstanding.
Ultimately, it means Ninty doesn't see a better use for their cash. Buying back stock helps keep the stock price up, which is the basic purpose of a publicly traded company. So, if Ninty has excess cash, but doesn't see a good way to invest it in the business, returning it to shareholders either through a dividend or buy back is the only way to go.