Good point - It is probably not as clear as it should be. I shall update the article.

It increases the 'potential' readership of your article, nothing can arbitrarily increase your readership.

I did a quick manual analysis of one of your stories with the following tags (followers in parentheses); Food (1.5M), Brain (1.2k) - which are good topics for your article.

The following tags - Gut Health(43), Probiotics(20), and Prebiotics(4) have a combined following of 67 people (You can see the details for the prebiotics topic here: https://medium.com/tag/prebiotics)

So, the last three will be in peoples' feeds for the those that follow the topics - but it is only 67 people.

Maybe you are specifically targetting those people - in which case - they are great topics.

If you were looking for larger potential readership, the suggested/recommended topics from the extension would be (with followers in parentheses):

Cooking (600k)

Diet (1k)

Health (2.3m)

Life (1.8m)

Mental Health (2.2m)

Neuroscience (300k)

Nutrition (100k)

Psychology (2.6m)

Recipe (100k)

Science (2.7m)

Self Improvement (2.5m)

Technology (4.9m)

Travel (2.3m)

For example, using 'Health' instead of Gut Health' will add another 2.3 million potential readers who subscribe to that the feed (versus 43 for Gut Health).

Whilst not necessarily an exact recommendation - it gives helpful hints to maximise the potential readership - and all of them have more than the number of followers for the last three of your topics.