Because space in mobile is limited, we should not ignore the features and we have to provide them to the users in different ways. For example, it is not possible to display navigation on mobile phones, but we don’t hide them. Today, a large percentage of users only work through mobile.
I have already reported an example of this problem:
List of post replies in mobile theme
In our society, many people are friends with each other and when a topic is created, they want to make sure that their friends visit the topic. In many cases, they try to mention their friends.
In addition, it can be sweet for the OP, it is interesting to know how many people are reading the topic you created.
It also greatly solves the problem of displaying all online users on the site. For example, if we have 10000 people online, it is not possible to display all of them at the top of the site, but it is easier to display online people in each topic.