Here's why:
* It modifies a verb, adjective, or another adverb. It tells us *how* something is done or said.
* Example: "He obviously didn't understand the instructions." (modifies the verb "didn't understand")
* It can also act as a sentence adverb, expressing the speaker's attitude or opinion.
* Example: "Obviously, we need to make a decision." (expresses the speaker's belief that the decision is necessary)