In short, an SSL certificate is essential for both your site and its security. SSL stands for Secure Sockets Layer and an SSL certificate makes a secure connection between a client and a server. The certificate uses two keys to encrypt data - a public one and a private one, making it more secure.
Sites that have 'https' in the address bar have valid SSL certificates, and it means that information shared within the site is secure.
The most main thing to know about SSL certificates is, if you're running a business, you will need one.
From January 2017, Google started to classify sites that do not have SSL certificates as being 'non-secure'; this means that if your site collects information such as passwords, cookies, or payment details it is imperative to ensure that you get an SSL certificate if you don't have one, or to make sure it is updated if it you already do.
Google has done this to make web users more aware of their security, and also due to the 'http' address being seen as a neutral indicator of a site's security - and the fact that web visitors can become de-sensitised to warnings about website's lack of security.
Even if your website does not collect sensitive information, Google recommends adding an SSL certificate to your website, as all browsing information can be altered or corrupted by third parties or intruders, in the process of downloading data between your site and the browser of your website visitors.
Thus, the certificate is important not only to maintain your site and brand integrity, but also to protect the information of your customers - potential or otherwise.
While your site's integrity is of the important, the final reason to obtain an SSL certificate is that it is also vital for updating new browser features, in particular, progressive web apps which integrate desktop browsing experiences with their mobile counterparts as the internet of computers and mobile phones begin to assimilate and become more uniform.
These reasons should be enough for you to consider the security and integrity of your site.