What is the benefit of forcing a site to load over SSL (HTTPS)?

HTTPS does not just provide secrecy (of which you are doubting the value, though there are good reasons for it still) but also authenticity, which is always of value. Without it, a malicious access point/router/ISP/etc. can rewrite any part of your site before displaying it to the user. This could include:

• injecting ads for your competitors
• injecting ads or annoying widgets that make your site look bad and harm your reputation
• injecting exploits to perform drive-by downloads of malware onto the visitor's computer, who then (rightly!) blames you for it happening
• replacing software downloads from your site with ones that have bundled malware
• lowering the quality of your images
• removing parts of your site they don't want you to see, e.g. things that compete with their own services or depict them in a bad light
• etc.

Failure to protect your users from these things is irresponsible.

"nothing secret on the site"

...According to you. There migh be a perfectly fine reason someone wants a secure connection. It (partly) creates privacy:

My admin can see that I'm browsing some picture site on my phone via url, but he can't tell if I'm watching pics of cute cats or hardcore porn. I'd say that's pretty damn good privacy. "a content" and "the content" can make all the difference in the world. – Agent_L

You might think it's insignificant, or maybe it's not a big deal now but could be at another point in time. I am a firm believer that no-one apart from me and the website should know exactly what I'm doing.

It creates trust. Having the padlock is a sign of security and it can signify some degree of skill regarding the website, and thus your products.

It makes you less of a target for e.g. MitM attacks. Security increases.

With initiatives like Let's Encrypt, which make it a lot easier and free, there aren't many downsides. CPU power taken up by SSL is negligible these days.

You get HTTP/2 support, the new web standard designed to significantly improve website loading speeds.

Because browser makers have chosen to support HTTP/2 only over HTTPS, enabling HTTPS (on a server that supports HTTP/2) is the only way to get this speed upgrade.