Foci

As written in the default rules, a spellcasting focus or component pouch removes the need for material components of any spell, unless the material has a gp cost specified or is already consumed.

Half and quartercasters gain an spellcasting focus or component pouch as relevant to their class.

Spellcasting foci and holy symbols draw on one's internal reserves of faith or magical power, and component pouches contain a small amount of every spell's component ground into a white powder. These are just enough to power any given spell, but not as much as pure components.

Pure Components

Pure components, a spell's specified components which are gathered from the world via exploration or purchase, are more potent. All pure components are consumed when used. If a spell's components are already consumed, they essentially require pure components and cannot be boosted by them.

When a pure component is used, the spell can be cast as if it was one level higher than the slot used, even if the spellcaster cannot cast spells of that level. If no benefits are gained from casting at a higher level, the caster may choose another benefit, such as a longer duration, additional target, or increased spell save DC, as approved by the DM.

If a spell's pure component is especially easy to come by (for example, "a pinch of dirt" for longstrider), the DM may rule that it does not confer any benefits.

Why?

I believe these rules give an opportunity for characters to have a "shopping list" of sorts and gives some more things to do during exploration for players not typically too involved in that stage. While requiring pure components for every spell would be tedious, this provides an incentive to do so rather than requiring it.

The DM isn't expected to know all the spell components and describe when they're around, but the players are meant to be searching for these. For example, in a mine, a wizard might say "Oh, is there a lodestone around? I could use one for my Disintegrate spell." The DM may then request an Investigation check to find a lodestone.