Google tries to move the entire web to HTTPS, but you may wondering if it's really necessary. If you only have a blog or any other site that only displays static content, why would you use SSL? John Mueller and Pierre Far, both from Google, have the following answer:
Some webmasters say they have "just a content site", like a blog, and that doesn't need to be secured. That misses out two immediate benefits you get as a site owner:
1. Data integrity: only by serving securely can you guarantee that someone is not altering how your content is received by your users. How many times have you accessed a site on an open network or from a hotel and got unexpected ads? This is a very visible manifestation of the issue, but it can be much more subtle.
2. Authentication: How can users trust that the site is really the one it says it is? Imagine you're a content site that gives financial or medical advice. If I operated such a site, I'd really want to tell my readers that the advice they're reading is genuinely mine and not someone else pretending to be me.