Here's why:
* It modifies a verb, adjective, or another adverb. For example:
* "He looked curiously at the strange object." (modifies the verb "looked")
* "She was curiously calm in the face of danger." (modifies the adjective "calm")
* "He spoke curiously slowly." (modifies the adverb "slowly")
* It describes how, when, where, or to what extent something is done.
Let me know if you'd like more examples!