With and without www, with and without a trailing slash, secure and not: the same page often answers at several addresses. A canonical tag tells Google which one is official, so the page's reputation doesn't get split.
Google treats different URLs as different pages until told otherwise. If your homepage lives at four addresses, links and reputation may get divided among them, and Google picks a favorite for you, not always the one you would choose.
One invisible line per page naming its official address. It costs nothing visually and settles the question permanently. Most website platforms can set it; many small business sites simply never have.
Choosing the official form of your URLs, setting the canonical on each page, and making the alternates redirect to it. Checked by the free review, fixed in the $40 tune-up where the platform allows.