They are the main backers of the SoftBank Vision Fund, Uber's largest shareholder with a 16.3% stake. The sovereign wealth fund of Saudi Arabia also holds a direct interest of 5.3%.
"This will give them more dynamism in this type of investment," said Daniel Ives, managing director of equity research at Wedbush Securities.
"They were there when Uber needed it," said Ives, calling the Vision Fund a "pivot" that allowed Uber to stay together at a pivotal moment.
The Uber IPO will probably be one of the largest ever conducted by a technology company. The company is fulfilling its ambitions not only in carpooling, but also in the transport, restaurant and logistics sectors.
This would be good news for SoftBank, Son and the sovereign wealth fund of Saudi Arabia.
SoftBank expects its money to give Uber enough weight to keep its competitors at bay until the use of the app becomes a habit for most people, according to Bill Aulet, professor at MIT Sloan School of Management.
Once that happens, the platform will be able to add pilots and runners at any time, he said.
Uber is also still grappling with serious legal problems. In its IPO, the company quotes "a number of investigations, inquiries and requests for information from the US Department of Justice and others." US and foreign government agencies, "which she says could hurt the company.
For SoftBank, the promise seems to outweigh the potential disadvantages.
The IPO is a boost for the Vision Fund and will stimulate future fundraising efforts, said Steven Kaplan, professor of entrepreneurship and finance at Booth School at the University of Chicago.
SoftBank's bet on Uber is also covered by the company's holdings in other stimulus services, including Chinese companies Didi Chuxing and Grab of Southeast Asia.
– Seth Fiegerman and Sara Ashley O 'Brien from CNN Business contributed to this report.