Do you think marketing is important for non-profit organizations?

Absolutely yes, try to imagine an NGO without marketing, how could it get funds and donations?! going for a pure sales approach will sound something like begging.
All organization types need marketing, even if they don't offer a product or service, at the end of the day they will seek a public support or community cause, saying that thinking about NASA for instance.

Yes,

it is very important to exists and serve for a long time.

yes important to reach out to the prospective donors

Yes it is important for every organization that is doing anything for others

Marketing is a crucial function that no organization can do without even if it is not for profit organization.
Not for profit organizations should be creating unique value at the end of the day for their customers.
Marketing will help you make gap analysis to know the segments exists in your society and which one to target.
It will help you to analyze the competitors, and how to differentiate yourself from them because in this case you may be competing for funding for example.
Marketing only will tell you if the service you are offering or planning to offer are relevant to your community or not through marketing research.
Useful marketing models like AIDA model (Awareness, Interest,Desire, Action) may be more decisive in not for profit organization because you usually want to go beyond the awareness and want to raise interest, create desire and receive actions from volunteers or from the community in order to drive your organization towards success.
Integrated Marketing communication will make sure that you are delivering the same message across the different channels you use to avoid conflict and to preserve your credibility in the community you are serving