Here's why:
* Adverbs of degree modify adjectives, verbs, or other adverbs to indicate the extent or intensity of something.
* "Barely" indicates a very small amount or degree.
For example:
* "I barely saw him" (modifies the verb "saw" to indicate a very small amount of seeing).
* "The room was barely lit" (modifies the adjective "lit" to indicate a very small amount of light).
* "He moved barely at all" (modifies the adverb "at all" to indicate a very small amount of movement).