Some builds need nodes that need too high a level to get, making them respec-only builds.

The reason respeccing is so expensive, and even harder to do back in PoE1, is because we aren't meant to reach endgame with a character and just respect our way into the endgame states of different builds without going through progression.
We are meant to progress each build we do from level one to max, with respec being a fine tuning option.

But some builds heavily rely on a particular combination of functions, that in order to play through progression as these builds, you need to already have those functions in play. Which is to say, they need to be reasonably obtainable at a lower level.
"Reasonably" meaning that rather than just beelining for them through attribute nodes, you can also grab some regular offensive and defensive nodes.
If it requires too high a level to simply reach the functions, combination of, the build is based on, it can become the case that the only viable way to use that build is to not actually play through the game using that build, just respec to that build only after reaching it's effective minimum level, having been playing as something else.

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My main example of this, that I've Seen so far, is a Monk using Darkness.

Embrace the Darkness:
Removes Spirit. Base Max Darkness is 100.
Damage taken reserves Darkness before taking Life or Energy Shield.
Reservation lasts X seconds (Upgraded by other node).
+5 Darkness per level.

Grasp of the Void:
Gain 1% of Damage as Extra Chaos Damage per 20 Unreserved Darkness.

So Darkness protects your health and energy shield, but the build is driven by keeping your darkness full to get that damage. You want to protect your Darkness bar.
So the first two things you want on a Darkness build, are Mind over Matter and Eldritch Battery. Possibly also Chaos Inoculation.
You want to protect the bar that protects your health and energy shield, so you want to set up mana to be your health, allowing your Darkness to remain full while you take mana damage.
Following that, chances are you'd intend to add poison, as that will scale with the chaos damage your doing. Then of course you need chaos damage nodes, mana regen/leech nodes, and other basic offense/defense nodes.

If I did nothing but beeline MoM and EB, that would take 31 points.
Add CI it's 33 points, throw in Whispers of Doom because Hand of Cha for 34.
But then I would have no offensive nor defensive nodes. Can you imagine having no real nodes at that level?
But also the Darkness simply won't be that useful until I have MoM and EB.
Plus I will desperately need mana regen and mana leech to make up for it being my health.

Considering the requirements, I couldn't possibly justify taking Embrace the Darkness on my Darkness monk at first Ascendency. I ignored it and took Reality Rending instead.
Now that I have my 2nd ascendency, I could get through to Embrace and Grasp, but I'm still nowhere near being able to take MoM or EB.
Currently my monk is level 46. I'm about to be halfway to max level, and I'm nowhere near the point where I've even begun to be using or progressing with the build I'm trying to make.

Right now, I'm still just a physical damage, energy shield, spirt using monk, as if Embrace the Darkness didn't exist. Not the Chaos damage, poison applying, no spirit, Darkness wielding, mana leeching monk I want to be.
It's almost as though I'm playing Invoker even though I picked Acolyte, just because it takes so many levels before I even can spec into my Acolyte build.