SSL certificates aren't valid forever. Once they expire, web browsers will display an error even though the site is clean. The error might scare the website visitor. The visitor will close the tab or open some other website.
Some SSL certificates come with a long expiry time. Some expire after a few months. For example, Let's Encrypt SSL certificates are valid for 3 months.
If you don't check the SSL certificate expiry date on time and you fail to renew the certificate, browsers will show an error instead of the web page. The bounce rate of the site will be very high. Search engines will consider your pages of inferior quality and will demote its ranking.
To make sure this doesn't happen with your website, you must check the SSL certificate expiry date on regular basis with this tool.
Enter the URL of a website and click the check button. CrawlCenter will process the certificate and will display the date on which it will expire.
Some hosting companies renew their customer's SSL certificates automatically. In case your host doesn't support this feature, you can set up a cron job that will renew the certificate a day before or the day it expires. If you've purchased the certificate, you'll have to renew it by paying money.