The question of which port(s) to use in an application is up to the application developer. The choice of which applications to use is up to the business management. The application deployment is up to the software and server engineers.
None of the above has anything to do with network engineering, and this site is for network engineering. Before Network Engineering was created, network engineers has to ask and answer questions on Super User and Server Fault, and it was difficult to wade through all the noise of completely unrelated questions. Network engineers got together, proposed a site dedicated to network engineering, and jumped through all the hoops SE requires to get it up and running.
The network engineering community decided what would be on-topic here, and what would be off-topic, in order to serve the needs of network engineering professionals for business networks, which is the whole reason for this site.
Every SE site has rules about what you can or cannot ask on a site. For instance, Server Fault is more than happy to entertain network questions, unless you ask about a home network, at which point, you will find your question either unceremoniously closed, or the question migrated to Super User. On Stack Overflow, if you ask a PC support question, be prepared to get a lot of anger and abuse directed your way.
Stack Overflow does a good job of supporting network programming, Super User does a good job at supporting home networking, and Server Fault does a good job of supporting the management of information technology systems.
Yes, there is some overlap, but professional network engineers really want a place to call their own to support their needs without having to wade through questions unrelated to network engineering. Network Engineering would be deluged with home networking questions if the community decided to allow those, and the professional network engineers would look elsewhere. Some network engineers here also participate on other SE sites, e.g. helping home network users on Super User.
Why would SE bother having almost 200 different sites if each site wasn't dedicated to different topics? There could simply be one large SE site. Each site has a community which it serves, and an "ask any question you want" site would quickly become out of control and defeat the purpose of the separate site.
As a member of the community, you are free to propose adding or removing which topics are allowed, but the community as a whole will decide.
Just let people ask their question. If it is that offtopic nobody will
answer and the user will ask on other forum too.
Unanswered questions keep popping up forever, looking for an answer, and that can quickly get out of hand. Also, asking the same question on multiple SE sites (cross-posting) is strongly discouraged, and it will often get the question closed on all the sites where it was asked.