Along with what kp98 mentioned, they're looking for the next Facebook, Google, Amazon, Alibaba, Tencent, et al. It's mostly that simple.
The returns that those companies have generated have been something beyond extraordinary. For example SoftBank turning tens of millions of dollars into ~$125 billion in Alibaba. Naspers bought 1/3 of Tencent for $32 million and that turned into ~$180 billion at one point.
SoftBank is willing to take N% of that $100b BABA windfall and vaporize it trying to generate the next $100b in returns (they won't be able to most likely, as China's 15-20 year boom is a one-off event that can't and won't be repeated).
They loaded into eg Uber because it was thought to be the next Silicon Valley homerun (which it was for some of the very early investors).
Travis Kalanick has a successful track record as a founder, selling Red Swoosh for $19m to Akamai and then Uber. The backers are betting Cloud Kitchens is perhaps at least a $20-$25 billion business at some point down the road. I doubt any sane person thinks it's the next Facebook, however it could certainly, potentially, be a relatively large business.
So Saudi Arabia buys 8%-9% or whatever at $5 billion. They're betting that they can generate a serious return from that valuation level (4%-5% at $25 billion for example; and if they're lucky, more). They aren't trying to play early VC, where you put in $1m trying to get a 200 fold return. They're looking to considerably beat the returns they can get on their money elsewhere, such as in equity markets, bonds, or even domestic industry.
Also keep in mind SA money is mostly dumb money. They're not at all competent to ascertain whether Cloud Kitchens even has the theoretical potential to be a publicly traded $100 billion valuation company one day. They're rolling the dice hoping to catch the next homerun. For the Saudis there's always more oil money, until there isn't.