The description of WinObj says:
WinObj is a must-have tool if you are a system administrator concerned about security
What kind of security issues can it detect and what is the rough usage?
Actually this is a debugging tool so to find security flaws, you'll need to know reverse engineering, assembly, C, C++ really well. Just using the tool and not having required skills will not help you find security issues.
In other words, WinObj allows you to visualise the Windows Object namespace, which is otherwise hard to see. Therefore any security problems which create or alter objects in the namespace become easier to find.
The question is very general, but I will try to answer.
In fact it is not going to detect any issue itself. This can be use to display some particular set of info about NT object.
Therefore, first of all you have to know the security parameters of NT objects and their values, and then you will be able to use WinObj to quick access and review them.
Sorry for such general answer, but the question was not very precised also.