I expect it’s for situations where that individual may be called back to consult on operations. Also, I’m not sure your clearance ever really “goes away” unless you do something wrong.
Clearance becomes inactive. Even if you do something ‘wrong’ it is possible to become active again if they want you bad enough. Or in my case the ChiComs apply for it since they hacked OPM and stole my info.
Until SP explained it to me, I thought that by having clearance you could look at any info that pertained to your clearance level. Even after leaving your particular job. What you, and SP, said makes perfect sense. That being said, i’m sure that retired politicians NEVER see any top Secret info :)