Individual donations are great, but the development fund gives us a regular income, which makes it possible to plan for the future. Fund raisers are most often feature oriented; the development fund gives us time to work on stability and polish.
Fund raisers also take a lot of time and effort; time that cannot be spent on working on Krita!
Of course, Krita will always be free and open source: there is no need to worry that the application will one day switch to a subscription model. But every little bit helps, and with a small monthly subscription you will help Krita’s development to progress at a stable rate.