About the verb "To attend"

Your reply implies but does not explicitly refer to an object. What will you attend?

“Please attend the course.” “I will attend.” is a structured conversation. If you include the original email in your reply, it is clear that you are referring to “course” as the object in your own sentence. The parallel use of attend in the two sentences strengthens the implication of your unwritten object.

“I will attend.” If you send your reply without the original, the object is unclear, and your sentence is incomplete, giving at least a feeling of carelessness, or even of sullen acquiescence.