I was talking to someone who is involved in a nature conservation nonprofit recently - small donations go into the general pot of money for the organization to choose how to spend it. If you want to influence what the money is used for you have to donate a significantly higher sum. They said they like having many small donors because they can fund things that don't necessarily make a big splash in a press release but are important precursors to impact (e.g. researching what projects would have the most impact vs actually implementing a project).