Captain America: Civil War - 16 Things We Learned From The Directors' Commentary
5. The Bug Guys Were Added For Levity
Though there will always be some suspicion that Spider-Man was added for a cheap pop and Ant-Man because Paul Rudd is a God walking among men, the Russos and the writers actually have an excuse for bringing both in.
Apparently, neither character has an emotional connection to the darker elements of the narrative (hence people wondering why they were bothered to fight), and they could thus bring in some much needed levity. But why both of them? Well, balance...
“It was important to have one on each side and you wanted the tone to be able to open up on each side.”
One of Peter Parker's specific moments of levity was also down to Holland himself: he ad-libbed the "His friend was huge" gag at the end.