The second thing is, once you have the words, you have to arrange them artfully. In grammatical -- or judiciously ungrammatical -- sentences.
So it helps to know your grammar.
Less than with words, to be sure. You can't use "amble" without knowing it, but you can write "Jill's ambling brought her to the store within four hours." without knowing a gerund from the passive voice.
Still, besides the obvious advantages in letting you talk about the way your sentences are structured, and letting others economically point out your errors, there are stylistic advantages to knowing the full armatures on which you can arrange your words. Gerunds, infinitives, clauses (subordinate or superordinate), compound sentences -- it makes it easier to look at a page and think, this sentences are awfully similar, because you can recognize that they are all simple clauses joined with a conjunction. And considering how you can rephrase them is easier when you are aware of the possibilities.
I have known writers who defended "not simultaneous" structures ("He woke. Throwing off his blankets, he walked to the kitchen.") on the grounds that you need to vary your sentences. Knowing your grammar helps avoid this.
And it helps to run across them in their native habitat, in the wild. Reading widely helps show how all these lovely grammatical tricks can be used.
(More advice to follow -- here.)
