I'm just curious, is really font size helps seo.

Yes, font size is critical in SEO. Google is capable of fully rendering the design of a page, and the page's layout plays a role in ranking. For example, it knows if you make those H1 tags teeny tiny or that regular div tag a huge font. Additionally, if you make your font size too small, you'll get penalized for unreadable text.

Good to know. Most of the folk I work with are more concerned with usability so no one makes text tiny or huge. As such we don't get a penalty and it's not an effect we see.

Usability plays a big role in SEO as well. Google factors in time on site, bounce rate, etc. in determining how usable your site is and whether the visitor they sent from a Google search had a successful experience or bounced back to the search engine results and clicked the next result.

As for it not being a penalty, you have to be especially careful with the current mobile-first indexing. The font might look great on a desktop but be a little bit too small or ill-proportioned when on mobile devices. You can click on the Mobile Usability tab of the Google Search console and see which pages on your site get an error for text being too small to read or clickable elements being too close together when on a mobile device. Now that Google ranks sites based on how they appear in mobile, this is more important than ever.

Thanks for pointing that out. As to the desktop and mobile views, all the developers I know test on at least the desktop plus iphone, ipad, android phone and android tablet. For a time the developer I worked with would test on a Chromebook but between finding nothing over a year and so few Chromebook users (one hit a month) we agreed we could stop that.

The Chromebook at the office found a good home.

I think so, I did worked on redesigning an online betting site before and identifying to appropriate font size and type is important. Visitors of your website will read important information, so you might consider the fact text should be user-friendly.