Track campaigns and optimize conversions.
Without UTM parameters, all traffic from a platform (like Facebook) looks the same in analytics. Adding UTMs lets you differentiate between specific ads, posts, or emails to see which one actually drives conversions.
Generally, 'Source' (e.g., google), 'Medium' (e.g., cpc), and 'Campaign' (e.g., summer_sale) are the big three. 'Term' and 'Content' are optional but useful for granular tracking.