Here's why:
* It modifies a verb, adjective, or another adverb. It adds information or emphasis to the statement.
* It indicates an addition or continuation of a thought. It often signals that more information is being provided to support a previous statement.
For example:
* "The weather was bad. Furthermore, the roads were icy." (modifies the adjective "bad")
* "The team played well, and furthermore, they scored a winning goal." (modifies the verb "played")
* "The movie was incredibly funny, and furthermore, it had a touching message." (modifies the adverb "incredibly")