Their impact on oil prices today, and in the foreseeable future, is inconsequential.
And sure Uber says it 'actively aims to reduce the number of cars on the road' but that's just lofty marketing speak. They just want to sell you a taxi service. But Silicon Valley startups can't just sell you useful services and products, they need to change the world.
Uber's impact will be the same whether they invest or not.
Saudia Arabia needs to diversify its holdings and sovereign funding sources as oil is a very narrow and volatile vertical upon which they currently hang their hat (and almost all of their funding).
Saudia Arabia is like an investment portfolio where 90% of its income comes from a single stock. They need to diversify and hedge.
Saudi Arabia (and even moreso prior to the 1990-1991 war, Kuwait, which was among the sources of the tensions leading to that war) invest quite heavily in diverse investments with a focus both on growth and avoiding having fortunes too closely tied to oil.