It obviously depends on the project and the scale and duration of a donation, but plenty of open source projects have benefited from donations. Donations can and do pay for developers, as well, part time or full time. With LibreOffice, you can answer your questions directly by browsing https://www.libreoffice.org/donate/ and as a non-profit you can easily find more detailed reports: https://www.documentfoundation.org/financials-and-reports/