Q:

Is it necessary to leave comments about deleting comments? 

I have noticed a recent trend where mods are leaving comments about having deleted comments (example, example, example).  I think its great for obsolete, chatty, unnecessary comments to be removed, but does a comment need to be left in their place?  I feel like the comments left behind by the mods are almost as cluttery as the comments that were removed. 

A:

Sometimes yes, sometimes no. 
We actually only do this on a fraction of the deletions we make.  The crumb trail is usually left when we delete something that had content people may not have realized would be going away.  This is part of an education campaign because we had a lot of vocal complains about comments being deleted.  By leaving these markers we hope to show WHY things are getting deleted and have a few reminders around so that future commentors will understand what the purpose of comments is and use them more appropriately in the future. 
The other major part of that education campaign was this meta post

How can we improve our guidance about the comment practices? 

There were several other threads complaining about comment deletion that lead up to that. 
Please note that we do try to only delete content that actually IS obsolete in that it has been integrated into the post or discussed to a resolution or moved to chat or something.  However there is still a significant amount of people placing CONTENT that is not just comments about the post in comments.  As long as that is still going on it seems to be useful  to make a note of what is being done so that everybody understand. 

A:

Necessary?   No, absolutely not!   But we leave them as signposts so that people understand what happened when they come back to their question or answers and all the comments are gone. 
It's definitely not necessary.   
However, there have been several people who have been confused when we delete their comments.   Because of that, we leave little signposts and breadcrumbs to let them know what happened and why it happened. 
We want to make sure people understand our actions and so we err on the side of over-commenting rather than under-commenting.