Let's look at an all martial party. A 20th level battle master fighter, samurai fighter, berserker barbarian, and assassin rogue. No magic items.
How do they take down a Demilich (CR18, which should be an easy encounter)? It's immune to non-magical weapons.
CR4 Coatls could give them trouble. Again, immune to non-magical weapons,
The mummy rot from a CR15 mummy lord would be a death sentence if they can't find an NPC to cast remove curse on them (or simply can't reach them in time - no access to teleport).