Ever since 3E came out, the currently published editions of D&D have included rules for "leveling up" monsters to keep their power level in line with those of the PCs. So instead of graduating from battling kobolds and giant rats to battling bugbears and carrion crawlers, then moving on to trolls and chimeras, and then on to frost giants and purple worms, we instead get just an ever expanding roster of creatures to fight. But it's perfectly possible for a DM to have a war against ever increasingly powerful orcs from level 1 to level 20 of the game, if that's what's desired.
And this is a problem for more than one reason. For one thing, aside from the ability to now face some creatures that were off limits before, as a player you still feel like you haven't really gained in power. Your character has gotten more bells and whistles. Managing all the special abilities, feats, spells, and magic items takes more time and focus during games. Things slow down because of it. But you still need to wipe out ONE MORE nest of goblins.
Part of this problem is also that the DM is either too engrossed with their idea of the great goblin war or whatever, or else too limited in imagination to use other types of creatures effectively. They know how kobolds work. Just keep pumping up their hit dice as the players go up in level, and it will all work out fine! Or so they think.
But another problem that people might not consider at first is that this means, to experienced players, that they will never know just how tough an encounter with orcs is going to be.
I don't hear much bemoaning of "metagaming" these days, say compared to 10 to 15 years ago. Maybe I'm just missing a lot of the community chatter since I'm not really in a lot of RPG discussion social media groups and don't frequent any forums regularly. I do remember people championing this exact phenomenon because it helped to "prevent metagaming." But what it really does is render an important part of player skill irrelevant.
If players know, from having faced certain types of monsters before, whether earlier in the campaign or from previous campaigns, that helps them with risk-reward assessment. They can gauge the power level of their part, the type and numbers of a group of monsters, and be able to judge easily whether to engage or try to avoid the encounter. Just like the artificialness of "dungeon levels" helps a party decide on their level of risk vs potential reward, knowing the monsters is information that experienced players (and by extension their characters) can use to make decisions. And decisions are the heart of game play.
Now, if a DM is going to go the 3E and forward path, and try to run level appropriate adventures where the PCs are assumed to a) take on every encounter they come across and b) have a near guaranteed chance to win these encounters until the big bad at the end...which they still have a pretty good chance unless they make some dumb decisions or the dice are just not there for them, well for that DM it probably doesn't matter if the orcs have 1 hit die or 6. The PCs won't encounter the 6HD orcs until they're 8th or 9th level.
But in an open world game, or a megadungeon, or any other more old school player driven game, knowing the monsters is part of the player skill set that should not be ignored.
Now, this doesn't mean that there can't be an especially big and tough version of a normal monster, or that DMs should never introduce new monsters to the mix. It's important to shake things up now and then. But really, this works best if the players KNOW most of the regularly encountered creatures. The creatures they don't know will make them act cautiously until they know what they're up against. And really, a good DM should be giving clues when they put in those tougher than normal creatures.
So, my advice for DMs? Don't scale up weak goober monsters for mid to high level PCs unless there's a solid reason to do so. Telegraph that when you do it. There are plenty of creatures of all challenge levels that can be pulled from 50 years worth of the game. And no matter what system you're running, it's probably not that difficult to convert between rule sets. I've converted plenty of 3E creatures to BECMI stats. And back when I played 3E I converted old school monsters to 3E. It's not that hard.
[Yes, this post was inspired by an event in one of the the 5E games I play in (via PbP). We ran into an encounter with orcs wearing black chitinous armor. They're a lot tougher than normal orcs, and we're (6th to 7th level) getting our asses handed to us. But the GM DID give us clues that these guys were tougher than normal. I'm not faulting him! He did it right. But the encounter got me thinking about this phenomenon.]