What is the importance of the title in an essay?

The degree to which the title of an essay is significant depends on the audience and rhetorical situation of the essay. If you are writing clickbait for the web, the title should be outrageous and startling to draw clicks, but for more sober genres, a title is less crucial.
The main thing that a title does is give potential readers a sense of the basic subject matter and approach of the essay so that they can judge quickly whether they wish to read it or not. For example, an essay titled "Best Foods for Your Cat" would appeal to people who own cats but not to people who do not own cats. 
For academic essays, your instructors read your essays as part of their jobs, and thus whether those essays interest them or not is not an issue. Instead, the only really important thing about your essay title is that it is in title case and clearly indicates the focus of your paper. If your essay has a title suggesting that you will be discussing flower imagery in Hamlet, and you actually talk about plants in Macbeth for most of the essay, you may have points deducted for lack of focus. 
https://owl.purdue.edu/owl/purdue_owl.html