My approach to proposals has some similarities to publications, but there are some drastic differences. First, is that the goal of the work is to submit a novel idea for research with the hope of obtaining funding. This can include some preliminary data to convince the reviewers that you are capable of doing the work, but any data included is to a far lesser degree than a publication. The proposal reviewers aren't going to give you money if you've already done the work! Secondly, I think proposals require a more extensive literature review. You need to convince the reviewers that the work you are doing fills a critical void that will have a significant impact and, at the same time, demonstrate that your idea has intellectual merit. Therefore, like a publication, I conduct pre-work to have a thorough understanding of what has been done and develop the project idea. I also review funding opportunities to identify a good match for my idea. This requires help from my access assistants, and, thus, I have never fully done the proposal writing myself.
I think that it was necessary to go through the process of writing publications and proposals, so that I know the pieces it entails. It's important to do the grunt work when you are learning during graduate school and during a postdoc. It's not impossible to do it on your own, but it's important to adapt to the different stages of your career and optimize your time and your abilities. Although you might take a step back from doing the grunt work, you still need to check-in every once in awhile so that you can be aware of any changes in the process or requirements.