Here's why:
* Adverbials modify verbs, adjectives, or other adverbs, providing information about how, when, where, or to what extent something happens.
* "She has spoken" is a verb phrase in the present perfect tense. The verb "spoken" is the past participle of the verb "speak" and is the main verb in the sentence.
To make it clearer, consider these examples:
With an adverbial:
* She has spoken clearly. ("Clearly" modifies the verb "spoken," telling us how she spoke.)
* She has spoken for hours. ("For hours" modifies the verb "spoken," telling us for how long she spoke.)
Without an adverbial:
* She has spoken. (This sentence simply states that she has spoken, but doesn't provide any additional information about how, when, where, or to what extent.)