As far as my understanding of all this - by deferring $680M without interest, it knocks the annual value applied to the Competitive Balance Tax to ~$46M per year over the 10-year span of the contract. That's because MLB uses a net present value calculation for luxury tax purposes and since the deferred payments are not gaining interest, the value is lower now.
One of the benefits for Ohtani is that by lowering the CBT rate and only paying him $2M in salary per year over the next 10 years, the Dodgers should be able to field a more competitive team around him. As others have already mentioned, there may also be significant tax advantages for him if he's no longer living in California when $680 of that $700M is paid out.
He's already probably filthy rich to begin with and collecting an estimated $50M per season in endorsements, so deferring such a large amount probably doesn't put a dent in his lifestyle.
There's really no risk on his end as the money is guaranteed. Also read somewhere that the Dodgers have to start setting parts of the deferred payments aside as soon as 2025, so there's no chance they can default on the payment.