my question was off toped https://mathoverflow.net/questions/298576/eiπ-1-1iπ?noredirect=1#comment742859_298576

Now I clearly see no reason for this. Just like for having two distinct websites for this purpose.

1. It is not intuitive to feel that mathoverflow and math.stackexchane have different goal.
2. If possible, we should not read any rules to use a website
3. No reason for creating an extra entity website.
4. Add new tags if you want to distinct by goal.
5. Researcher mathematicians may find useful information on general abstract theory more often which may not even appear on mathoverflow.net

Thats things that seems obvious for me as for many-year user of stackoverflow.com.

What's the reason of such an unobviousity?