Here's why:
* It modifies a verb: It tells us how something happens. For example, "He scarcely finished his meal" tells us he finished it with little effort or time.
* It can modify an adjective or another adverb: "The room was scarcely lit" (modifying the adjective "lit") or "She walked scarcely at all" (modifying the adverb "at all").
Let me know if you'd like more examples!