The thing is, DMs are also game designers. The monsters in the books are just tools, a DM has to actually place them into a dungeon or other adventuring environment to make a game. And it can be useful for them to know what a monster’s combat function is.
No we are not except in the very broadest/loosest/poor definition of game designer. Yes, I would speculate that most DMs active on this forum are also game designers. But with nothing but experience tells me that the larger pool of DMs simple are not game designers. Since we know this forum represents a tiny fraction of the RPG players and GMs, I think it's more accurate to assume that we are the exception, not the rule.
Is that not reason enough? Making combat easier for DMs is a good thing.
There are many ways to make combat easier for DMs. And a whole lot of them are horrible ideas. Let's make combat easier for DMs; All monsters have the same attacks, saves and abilities, only their HP varies. All creatures have three actions; single attack, full defense, and disengage. All movement is the same, there is no terrain. There is no cover. There is no range, if you have line of sight you can attack. Their are no combat modifiers. There is no advatage or disadvantage.
There, the game is now really easy for DMs.
No, making the game easier for DMs is not a good goal in and of itself.
I mean, I guess you could try using a monster that was designed to be a mobile but frail damage dealer as a a tank, but I expect the results would not be ideal.
So every encounter must be ideal? Come now, variety and spice and all that. But, I did not say that monsters should not have things they are good and bad at. Nor did I say that such information should be easily accessible to DMs. I said making it part of the stat block was a bad way to implement it. There are quite a few others in this thread that agree with that statement, and have suggested other solutions.
Nothing saying you can't do the same tagging with other kinds of stuff. Take social interactions as an example. You could just as easily write up something similar for social roles. It's not about pigeonholing anything, it's about helping referees easily find appropriate content quickly.
Depending upon how you implement it, it can result in pigeonholing. There are other ways to share this info with DMs without making it fraught with many DMs assuming it is inviolate. (Such as the old monsters by terrain tables. Creating similar for roles would work easily and not risk the downsides. And I'm sure others have even better solutions.)
Ah yes, a classic problem of the DnD community: baulking at a tool that will help you build and run your games easier because you think it’s telling you how you should play
Bad tools are bad, even if their goal is desirable. There are better ways to achieve this goal that do not have the drawbacks of this method.