There are two main reasons to use an SSL certificate. The first is to protect people who use your website. It stops hackers from being able to eavesdrop, steal and even modify your users' data while it is being transmitted between the website and web browser. This might be financial, personal or confidential information. The second reason is to encourage people to use your website by having a trusted third party verify that your website is legitimate.
The only downside, apart from the cost, is that due to the encryption, your website will run a little more slowly.
