There is more to the rules of English grammar than we have covered here, but this should be enough to get us thinking about how to form our sentences in grammatically complete ways.
Remember, the goal for all of our writing is to clearly communicate our ideas to our readers. If our sentences are incomplete.
Well, as you can see, confusion follows, and confusion should always be avoided.
While grammar checkers are effective, we should not rely on them to "catch" our mistakes. Rather, we should be thinking about exactly what we want to say to our readers, and how we can express these ideas as complete thoughts in separate sentences.
Lastly, it is worth remembering that by writing in grammatically complete sentences we imitate the natural rhythm and flow of English speech. Writing, after all, came much later in human development than verbal communication, and grammar is merely an attempt to capture these interactions on paper.