Bill Gates is financially involved in a large number of startups mentioned in the article. Good PR = more sales = Gates' portfolio is better off.
The other reason is a little more conspiratorial. There is a growing trend of financial populism where politicians are clamoring for higher taxes on the rich (70% on $10+ mil). I think Gates is trying to mitigate this movement by showcasing how he uses his wealth for the greater good.
Many people in his position would be looking to try and make the world a better place for his children. Bill Gates however is simply technically strong and wealthy enough to do this through more direct means.
> One estimate puts extra annual revenue from Ms Ocasio-Cortez’s idea, which applies only to incomes above $10m, at perhaps $12bn, or 0.3% of the tax take.
https://www.economist.com/leaders/2019/02/02/how-to-tax-the-...
He's so astronomically rich he's barely human. What could anybody do to affect him in a way that changes the things he could do with the money he has?
He is right that a 70% top bracket won’t increase revenues by much.