Imagine trying to purchase something on a Website and getting a message that the secure connection has failed and the site is using an invalid certificate. Would you think twice about proceeding or even abandon your purchase? Probably, yes. That is what happens when a site has an invalid SSL certificate. It may be that the cert has expired or just wasn't setup correctly.
And what does this do to the user experience on these sites? Well, not a lot of good. It doesn't instill much confidence in the user; not to mention, it can inhibit the user's ability to access your site altogether. It also presents a security threat as the user's information may be vulnerable and prone to hacking and phishing scams which can become a liability for the company. It can also result in downtime for the site. In the end, both the company and customer loses. So, to prevent this from happening to your site, keep those SSL certs current.