One language where this is currently practiced is Verilog. To make things spicier, different vendors have different syntax for semantics-changing comments, often prefixed with the vendor's name.
One "benefit" of this is that an older tool not updated to include the new syntax embedded in the comments will remain blissfully unaware of its ignorance, instead of spitting an error message. Another "benefit" is that a programmer unaware of the syntax might delete or copy & paste such comments when doing massive editing, without realizing that they aren't comments. To achieve the latter, it helps if the syntax looks like English or, better yet, if it looks like commented-out code (and, come to think of it, it's not unlikely that it will look like at least one of these things.)
One "benefit" of this is that an older tool not updated to include the new syntax embedded in the comments will remain blissfully unaware of its ignorance, instead of spitting an error message. Another "benefit" is that a programmer unaware of the syntax might delete or copy & paste such comments when doing massive editing, without realizing that they aren't comments. To achieve the latter, it helps if the syntax looks like English or, better yet, if it looks like commented-out code (and, come to think of it, it's not unlikely that it will look like at least one of these things.)