Other Stack Exchange websites cover a wide range of possible questions one can ask about computers:

• software recommendations: http://softwarerecs.stackexchange.com/
• questions about running/using some programs: superuser
• questions about programming: stack overflow
• hardware recommendations: http://hardwarerecs.stackexchange.com
• …

In which case is it better to ask on retrocomputing?

Not covered somewhere else:

• computer history (I guess most such questions wouldn't qualify on history Stack Exchange)