Writing by the sentence is kinda like buying by the piece. You may need a whole lot of something, but you pick each individual one that you want.
Writing by the sentence means crafting each sentence. It means making sure it comes from the one before and leads to the one after. It means, for a grant proposal that each sentence works, does what it needs to, and not much more. It means there are No. Wasted. Words. If anything irritates me its having 2-3 sentences in a row that say the same thing.
This doesn't mean that the same information shouldn't show up in multiple places. When I had the consultant who got me funded on a proposal (here's the post based on his letter of support for me), I mentioned his collaboration in several places (the significance, at least two places in Research Design, and certainly in Vert Animals, as that was his expertise).
Writing by the sentence is one way to achieve making every sentence work hard for you. In fact, making every word work for you. The subject and the predicate need to convey information and not be place holders.
One of the problems of writing by the sentence is that you get bogged down. You get lost. You lose track of the forest, let alone the ecosystem. That's why Darwin discovered outlines. Do the outline first, and then fill in the sentences.
There is much more to say on this, especially examples. But! too many things happening this morning.