It's actually not entirely that. Google appears to have some kind of unpublished "quality score" that is applied to emails received in a Gmail account, and it uses this score to determine if the message will appear on the inbox tab or in the promotions tab (or worse, the spam box).

One big part of raising this mysterious and possibly just a myth "quality score" is to increase your emails open rates and especially your click-to-open rate. Basically, the more people you get to open your email and then to click through to your website, the better that Google will think your emails are performing and the more likely they are to appear in the inbox in the future.

Plus, clicking through an email provides tracking information not only to Google, but also to the site you clicked through to, which helps them with their marketing campaigns or whatever else they are doing with their tracking info.

There is an entire pseudo-science that email marketing people use in attempts to increase deliverability and "inboxing" in Gmail and a lot of it just comes down to "get more people to click through to your website", and it does appear to help, at least anecdotally.