Deleted comments are hidden from users that aren't staff members or the user that posted it. Basically it's so you can't post some horrid racist/political/personal attack/generally horridly derp comment that would normally get you in trouble then delete it later and claim it never happened. Or, a mod can't go through and delete a bunch of comments without a trace, and they can be undeleted if need be.
Deleted comments are hidden from users that aren't staff members or the user that posted it. Basically it's so you can't post some horrid racist/political/personal attack/generally horridly derp comment that would normally get you in trouble then delete it later and claim it never happened. Or, a mod can't go through and delete a bunch of comments without a trace, and they can be undeleted if need be.