Unfortunately, there is very little available data on how often users interact with and click on sources in ÄF boxes. This is probably because Google does not display this information in Google Search Console and there is no other reliable source for this data.
However, it is safe to assume that Google would not display the “Similar Questions” if users did not use them as well.
Most users who use the “ÄF box” will probably not click through to the original source. Here, however, one should keep in mind that the interaction does not produce a visitor on one’s own website, but it does produce a high-quality brand impression. After all, Google chooses its own website as the source for a relevant question.
It can therefore make sense to optimize your own website for the “Similar questions box.
So similar questions on Google are quite a relevant SEO tactic.