The short answer to the question above is VERY!! I’ve already written a couple of articles about this, but it bears repeating:

How the proposal looks is extremely important, especially when you consider a couple of important facts.

Number one, studies have shown that almost half of your readers—in fact, half of the human race—takes in and processes information better if it is presented visually, in concepts that are rendered graphically. The other half processes information more efficiently when it’s presented in text. That’s the way people’s brains are wired.

Yet, most proposals are primarily text-based. In fact, I’d say that most proposals—most business documents, for that matter—are 90% text, and the rest pictures or other graphical elements.
Look at this from a purely information-processing perspective. When you write a proposal that is mostly text, you’re making it easy for just one-half of your readers to take in what you are trying to tell them. The other half—the 50% that has a harder time processing text—is being ignored. In essence, you’re making it harder for them to get what you’re saying, which will make it harder for them to buy what you’re selling.

Okay, that’s one fact that has a bearing on how important the visual presentation is. Here’s another:

People today are extremely busy. They don’t have time to read every word of every page that’s in every document they have to read. So, what do they do? They scan. When they turn a page, they look for something on that page that can give them an idea as to what the rest of the words are all about. If they don’t find that there, they skim, looking for something to grab their attention.

They’re looking for some kernel of interest they can grab onto that will compel them to read further. Graphics offers them that. A graphical element will help them understand what you’re saying so they won’t have to read the whole page. Oh, maybe a few will read the whole page, but I’ll guarantee you that most of your evaluators will not.

You can help these busy people by giving them graphics that clearly capture the essence of what the words are saying. And you can help the people who process information easier when they see it graphically by going to the trouble of giving them clear, well-thought out images that make your point.