The primary function of the wp-login.php file is to gain access to your WordPress Dashboard. While the security of this page does vary, having a strong password is the first line of defense in keeping malicious entities out.
Outside of using this as just a login page, did you know you can modify how it looks? When creating websites for clients – especially larger ones – they may be interested in having a custom login page that is relevant to their employees and customers. To achieve this, you could pull from your understanding of PHP editing or leverage the power and ease of use that plugins offer. Please note that if you opt for PHP editing, when your WordPress installation updates, it may undo some of the edits you published.