Donations are a monetization model.

Servers and infrastructure cost money. People's time costs money. You need enough donations for it. If you don't, you have to strategize to get more.

Brian action established the signal foundation with an initial $50mm grant.

* $50MM loan

The key distinction being that loan will need to be repaid.

> The initial $50M in funding was a loan, not a donation, from Brian Acton to the new nonprofit Signal Technology Foundation. By the end of 2018, the loan had increased to $105,000,400, which is due to be repaid on February 28, 2068. The loan is unsecured and at 0% interest.[5]

https://en.wikipedia.org/wiki/Signal_Foundation

It's probably some tax dodge or something.

Donations, or an interest free $100 million 50 year loan from the founder of WhatsApp.