It is necessary if you want to build managed C++ assemblies which try to
overcome the "Mixed DLL Loading Problem"

See: Mixed DLL Loading Problem
http://msdn.microsoft.com/library/en-
us/dv_vstechart/html/vcconMixedDLLLoadingProblem.asp

Or in general: If you build with /clr and /noentry

In short it does the following:

<quote>
The compiler adds a call to an internal C runtime (CRT) function called
_check_commonlanguageruntime_version to check if it's version 1.1 of the
CLR. However, since you have removed support for the CRT by using the
/noentry linker switch, the linker will complain that the CRT is not
accessible by complaining that it can't find the symbol _main. The way to
get around this problem is to link with the nochkclr.obj file provided in
the Visual C++ .NET libraries folder or to compile with
/clr:initialAppDomain described in the next section.
</quote>

See: Language Enhancements and Simplified GUI Development Enrich Your C++
Apps
http://msdn.microsoft.com/msdnmag/issues/03/03/VisualCNET/