Captain America: Civil War is filled with superheroes, but Spider-Man's introduction to the Marvel Cinematic Universe is one of the most crowd-pleasing moments of the film.
The rest of this post contains spoilers for the film.
The character is first seen as Peter Parker, and we establish that he's new to the world of wearing tights and fighting crime. The film takes its time with the character, and we get to know this Parker a bit and learn about his motivations before he shows up for the epic battle between Avengers that happens near the midpoint of the film.
After that battle he's sent back to his "normal" life, although the post-credit stinger teases his return. You could remove the character without changing the movie at all.
Spider-Man makes the film stronger, especially due to Tom Holland's winning performance of the web-slinger, but why wasn't he given more to do?
"When [Civil War screenwriters Christopher ]Markus and [Stephen] McFeely started working on Civil War ... Marvel had not yet finalized its deal with Sony to share the rights to Spider-Man, who makes his MCU debut in the film," Vox explained. "That meant they had to start writing without knowing whether he would end up in the final script — and thus had to build a story that would allow him to be replaced if necessary."
This explains why the characters feels a bit secondary to the film's main story; the narrative had to be able to work if you removed Spider-Man completely.
"We were flexible enough so that if somehow the improbable deal between Sony and Marvel didn’t happen, we weren’t left having to redo the entire structure," McFeely told Vox.
They described the story's structure as "modular," and Spider-Man was one of the modules that could be added or removed without substantial changes to the story. Spider-Man makes the film arguably better, and certainly sets up future movies with this new take on the character, but he wasn't necessary to Civil War by design.