I think that strongly depends on the country and project you're looking at. Sometimes, "help" is a synonym for buying access to resources via corruption. And yes, private institutions are doing that as well - maybe even more open than states. The oil companies have their reputation for a reason.

And sometimes it's really help. You mentioned the Gates Foundation, which is a good example for an altruistic private institution. But than there's also the public funded programmes of the UN, or single countries' initiatives. E.g., the Republic of Austria funded the construction of water power plants in Bhutan, whithout expecting anything back (my father was involved in the project as an engineer).