There are two big reasons that clear writing is important. First of all, it lets as many people as possible understand what you have to say. Secondly, writing clearly forces you to think clearly; it improves the quality of your ideas.
Using jargon-free writing appeals to the largest possible audience. Experts in your field can still comfortably read your reports, but non-experts can understand them, too. It takes a little more work to find understandable terminology for technical ideas, but doing your best is well worth it. Your donors, staff members, and the people you serve probably don’t have the background to read a jargon-dense article, and these are your most important audiences. There may be a few highly-targeted documents that need to be heavy on technical terms, but even then you can still write well.
Using jargon-free writing also forces you to think about what you’re saying. Jargon makes people’s attention – even your own – slide away. If you write that you are going to “include stakeholders in decision-making,” you don’t have to stop and think about who, exactly, you will include or how you’ll make them part of your decisions. Jargon is an obstacle to good planning. Clear, specific language, on the other hand, leads to clear, specific thinking and plans.
(Here’s a tip: if you are so far into the belly of the beast that you can’t tell what is jargon any more, read your writing out loud. Anything that stumbles off your tongue should be removed.)