Introduced in WCAG 2.2 as success criterion 2.5.8, the target size requirement exists because small, tightly packed tap targets are difficult for anyone with tremors, limited dexterity, or larger fingers to activate accurately, which is a common problem on mobile interfaces with densely packed icons or navigation items.
There are some built-in exceptions, including inline links within a sentence of text and cases where an equivalent larger target is available elsewhere on the page, so the rule targets standalone controls like icon buttons and toggle switches rather than every clickable element.