I'm doing an ACT practice problem, and I'm confused.
In the sentence "Thousands of visitors from around the world travel to Siena during the summer, not only to witness the exciting race but also to attend the after-parties thrown by the locals." there is a comma before the not only. Why is there a comma?
The ACT's explanation is: "The second half of the sentence is an incomplete idea and must be linked to the complete thought." Note the question is asking whether there should be a comma before the not only.
If you were to remove the not only, I think the sentence would become "Thousands of visitors from around the world travel to Siena during the summer to witness the exciting race and to attend the after-parties thrown by the locals." In this case, the comma is removed, so why is there a comma when not only is added?