By the house grows a poplar. Each spring its branches shoot for the sky, eager to extend the tree’s rocket-shape.
Try writing a third sentence about the poplar.
Did your sentence use poplar or tree? Did you feel clumsy having to repeat a prominent word that was already used? Or perhaps you went for an unambiguous application of the pronoun (Its roots dig further down into the gravely earth …)? What would you do for a fourth or fifth sentence?
If you’re wondering why word-variation matters, consider the example without it:
By the house grows a tree. Each spring the tree’s branches shoot for the sky, eager to extend the tree’s bullet-shape.
Aside from losing the specificity, we’ve lost a solid, well-formed image to the inane hammering of a word.
You usually notice that you’ve referred to something in the same way across multiple consecutive sentences during a rereading of a draft. Then comes the question of substitutes. My example above is fairly prototypical for common nouns: there is at least one other word which can serve you immediately (poplar) and one pronoun you can seize on (it). If those are not enough, then the problem lies with uniform (and therefore uninteresting) sentence structure, and it’s a matter of reworking from the elements up.