Attrition is the slow bleed of resources across a day of encounters.
An ogre warband costs the wizard a spell use. A troll ambush forces the cleric to spend serious healing magic. A trap triggers and the thief takes 10 HP. None of these solo encounters should be lethal. But stacked together before a final rest, they create desperate circumstances.
By the time the party reaches the frost giant, they're half-spent. Even a fight many would rate "medium risk" becomes deadly when it's the fifth or sixth challenge of the day. This is where high-level play truly shines - it stops being about single battles and starts being about whether the party survives the entire gauntlet.