Newer versions of .Net and .Net core has have removed and/or changed “Code Access Security” (CAS) since this question was asked.
I was the development lead on a project to get JITC certification (US Department of Defense) for a .NET based solution, and the CAS settings were scrutinized very closely during the certification testing.
Like most of the other certification requirements, the code could only use the privileges it needed to work and no more.
If you are planning to get security certifications CAS can definitely be important.