From my experience, these organizations throw parties because they have to. This is how non-profits fundraise. I don't think there is a non-profit organization on the planet that has a significant impact and is funded only by small individual donations. Money is raised by throwing a party, presenting all the great work you're doing, and getting rich people drunk so they either give you right there and then or secure you a meeting with someone who will on the spot.
These events are funded by your small donations.
This sounds like a horrible amount of overhead, and it is, but that's how the world works. Most NGOs will say they have very small overhead, something silly like 5%, but that's because they don't count fundraising as overhead, or they put fundraising events into "programming" which again doesn't count as overhead.