As a DM, I occasionally throw in an encounter that is way too easy, so the players can enjoy the cakewalk, ... also an encounter that is way too hard that they probably need to run away from.
That I agree with completely. Which is, again, why powergaming is ultimately pointless. The problem is that powergamers do not understand it. They think the DM has to be fair and abide by rules about encounter difficulty, that he has to respect the technical power of the characters. He does not, as a DM my ultimate aim is for the players to be happy about the game and the story told, and at our tables it has, in the end, little to do with technical power, but a lot to do about roleplaying, thinking as your character and finding solution as your character, not about using a technical power.
And getting a +1 is not going to get you out or make you win unwinnable encounters. It's only actually going to annoy the other players as you want to show it off over the world and over them, and as you want to set up situations in which you think you will win using your ultimate power combo. I've seen it many times, powergamers inciting fights or creating situations in which they think that they will shine.
But that only works in a technical game. Again, there is nothing wrong with this, if all the players at the table want to play a power game of fighting (and of course this does not prevent roleplaying to happen too), it's perfect for that table.
But at mixed tables, and even more at tables like ours where story matters more, that behaviour will at best annoy other players whose plans and roleplay are torpedoed by the need to take technical action, and at worst will get the powergamer and possibly the rest of the party in bad trouble.
Typical examples in two of our major campaigns these days. In the Avernus that I'm running as a sandbox, all the adversaries are more powerful than the PCs by far (these are devils and other fiends, and in hell), so they are slowly and bit by bit building a small army of (untrustworthy, but it's all the more fun that way) allies. Going for any of the objectives that they have straight on thinking "the DM will balance the encounter difficulty to make it barely winnable because we are brilliant players with powerful characters" will not work. And the same in our Odyssey of the Dragonlords campaign, where our adversaries are gods, dragons and titans, only this time our allies are more trustworthy.
It helps with verisimilitude.
It also makes the players and their character think twice about taking the combat option (because they can't be sure that they can win, and I use the Worf effect a lot as a warning), but really rewards them if they take it well (by making the fight a cakewalk if they have investigated how to win it painlessly).