(Donations are done to Open Source Collective 501c6, a US non-profit that collects and holds funds on the behalf of the curl project.)
To ensure that the curl project continues to thrive. Even open source projects has expenses! You can use curl forever without paying anything, but maybe you think you have got quite a lot and feel you could contribute a little back?
See also Mary Gardiner's excellent guideHow To Pay for Free Software.
Donate your time. Answer questions, contribute code, review patches and help out. Or let one of your employees do this for the benefit of the project!
If you cannot donate time or skill, consider donating a small amount of money. Maybe even do it in a recurring manner?
Anyone donating 100USD per month, or more, is considered asponsor and could get listed on that page - assuming the team agrees.
Donated money is strictly and only used for project expenses, in particular thebug bounty and the curl annual conference seriescurl up.
Please note that sponsorships and donations are exactly that: donations to the curl project. They are used to help and further the project as the project leadership deems best. No goods or services are expected or promised in return. Requests for refunds for such purposes are rejected.