Single-issue comics almost always have extra material in them, as an incentive for folks who read them serially as they come out. (It's generally more expensive when one reads in single issues, so the publishers find any hook they can.)

In particular, most single-issue comics have letters columns--obviously more important pre-net. Kelly Sue deConnick's amazing Bitch Planet, for example, has brief essays from a wide range of feminists in each single issue, but they're not in the TBPs (2nd one debuts mid-March)
https://imagecomics.com/comics/series/bitch-planet