To name just a few: money, jobs, better roads and ports, knowledge, and new ways to grow the economy.
Many of the Belt and Road countries are developing countries which need to reduce poverty, improve infrastructure, industrialize, or balance their economy. China can help with its capital, experience, expertise, and talent. The supply-demand match opens great opportunities for collaboration.
This kind of cooperation did exist between China and other countries before the Belt and Road. But there is no doubt the gravity of future cooperation will be tilted to the Belt and Road platform, which has absorbed many existing projects.