Here's why:
* Nouns name people, places, things, or ideas. "Desk" is a piece of furniture.
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.
For example, you could say:
* "I am working at my desk." "Desk" is the object of the preposition "at" and is a noun.
* "I am working diligently at my desk." "Diligently" is an adverb modifying the verb "working".