Regardless of the incredibly awful title, I believe this is a request for a useful feature. Since a couple people seem to have misunderstood (or maybe I have?) I'll try to explain with some images. Both of these are situations I've specifically come across recently which would be improved with this feature.
The first is "I want to read a new manga in X genre/theme". For example, I like reading crossdressing manga. If I'm trying to find new crossdressing manga I use the search and get these as the first 20 results:
Counting them up, I have 13 manga that I have already tried, and only 7 that I have not (some of which have no chapters in my language, but that's a separate issue). Being able to filter to only manga without a following status would make it a bit faster and easier to discover new manga of a particular genre or combination of genres when I already follow a large number of them.
Another situation that happened recently is I wanted to find a specific manga that I knew I followed, but I couldn't remember the title. I remembered it as "Henchman of an evil organization tries to take over a fantasy world". From that the only tags I can confidently use are "Isekai" and "Fantasy", which give me these results:
Here, the manga I was searching for was "Sentouin, Hakenshimasu!", at the 35th place on the list. If I was able to limit the search only to manga in the "Reading" status would have saved me a bit of time and reduced the chance for making a mistake when going back and forth between title and status.
It's not a matter of "things that cannot be done without this feature", but in both these cases (again, things I have specifically tried to use Search for recently) being able to filter or sort on the following status would have made it quicker and easier.