Why was callen shot on NCIS?

In the NCIS: Los Angeles episode titled "The Guardian," Agent G. Callen was shot as part of a larger storyline involving an investigation into a terrorist cell and the pursuit of a dangerous individual connected to his past. The incident served to heighten tension and drama within the narrative, revealing the risks agents face in their line of work. It also explored Callen's character and history, showcasing how his past continues to affect his present.