to artificially increase the number of members.
You can have a forum with thousands of members who write nothing. There are forums with far below than 100 members who are fully active.
And what kind of impression I get as a visitor / interested, if there are 10'000 members in a forum, but hardly any topics and posts! The same, if all contributions contain the same user name apparently all from the operator.
In any case, I remove the long time inactive with no post. Especially the one-day flies.
The topic, the content, the daily activity especially writing, the name of the Internet address, that is important to make a forum interesting for visitors.
If, for example, I open a forum for extremely rare diseases, I have to expect that I will not have too many members. But that still does not mean that there will be no activity in the form of new contributions.
It may also be that you have thousands of visitors daily. This shows me that the forum is still interesting despite the lack of new posts. It is then just a kind of public archive.