You are writing with words (duh!). Keywords are those words that attract users of websites use when they are searching for something. This something could be a product or a service. As a result, productive copywriting uses as much as possible of the keywords that the target visitor uses.
In this respect, it is good to mention that the so-called keyword-stuffing is not allowed by Google. Keyword-stuffing is the bad habit of naming the keywords all the time.
As an example, when we write about online digital marketing, and we mention these three words more than once or twice on a page, Google will give us bad marks. And with these bad marks, it will be tough to rank high.
Also, keep in mind that when you are copywriting, you write for the user. And the user is not a search engine. The user should be able to absorb your stuff easily. If not, then we are back at the hindrances, as mentioned earlier, and the user might wish to leave too soon.