Here's why:
* It modifies verbs, adjectives, or other adverbs. It tells us to what extent or degree something is true.
* Example: "I especially like the red one." (modifies the verb "like")
* Example: "This is especially important." (modifies the adjective "important")
* Example: "She ran especially fast." (modifies the adverb "fast")
* It can't stand alone as the subject or object of a sentence. It needs to be connected to a verb, adjective, or other adverb.