Armor reduces damage. Plain and simple.
More armor reduces more damage. Now it get's interesting, because the damage reduction is not linear but logarithmic.
This means something like this:
One axis stands for armor, the other one for damage mitigation in percent. See more details here.
You can see that at a certain point (or rather intervall) more armor becomes less effective. We are talking about diminished returns. This leads to players not bothering increasing armor above a certain value.
However, Queen of Thorns and Erebus both have skills that reduce armor. QoT's Spikes lower armor by a whopping 1500 and Erebus' Bite manages a decent 700. And both attacking together drop armor by 2200 points which can seriously lower the survivability of affected opponents ... especially if said opponents neglected stocking up on armor, because armor can drop below zero.
The following table shows us what a Erebus/QoT can do to a demigod's defense.
The values in the Spikes/Bite columns represent the extra damage dealt by melee attacks to a demigod with the armor specified in the first column.
Some constellations obviously can never happen, so stop eyeing the corner values.
Some reference points:
Natural Demigod armor at -
Level 1: 220-475
Level 4: 306-571
Level 7: 372-667
Level 10: 438-763
UB has the highest armor, Erebus starts lowest but Rook "undertakes" Erebus before level 4.
Buyable armor for -
400g: +600 (Scale Mail)
1500g: +750 (Nimoth Chest Armor)
3250g: +1050 (Armor of Vengeance)
5200g: +1200 (Groffling Warplate)
There are more items providing armor, but I only wanted to list some significant examples.
If we look back at the table then we can roughly find the armor a DG might have at the moment you poke him with spikes of a certain level. Level 1 DGs enjoy about 14% extra damage from a Level 1 QoT if they forgot to buy armor, about 9% if they didn't. Not very promising, given that QoT isn't exactly a big damage dealer.
However, should they continue to neglect buying armor while QoT levels up, then the effects of higher Spikes become quite noticable: About 28% at Spikes 2, 46% at Spikes 3 and up to 69% at Spikes 4. Noobs might forget to buy armor ... or Rooks that successfully defended a lane for a very long time without retreating. But those that didn't forget probably have about 2000 armor by the time QoT gets Spikes 4, which reduces the bonus damage to about 28%. Still nice.
And in case you team up with Erebus, you get even nicer figures. 53% extra damage against a bitten and spiked DG surely makes a difference. Remember the Oak skill Penitence? DGs hit by that suffer 16% extra damage for a while. DGs have to stack up armor until they reach 3300 to bring the effect of Spikes 4 down to that level.
However, armor reduction gets less effective the longer you play, since most players automatically buy the occasional piece of armor. The bonus provided by those skills get lower and lower the further the game progresses, which probably accounts for the QoT-sucks-late-game observation. Indeed Spike has it's golden age in the early mid-game phase where you can surprise a low armor DG with a fresh level 3 spike, boosting your melee damage against him by nearly 50%. With lots of minions in a creep lane you can wipe out the enemy creeps in an instant and let your creeps and minions concentrate on the unsuspecting victim, resulting in a quick kill.
Sadly, that's about the only phase in any game where you can expect those quick kills.