Advertisers want to know how effective their ads are. They want to see if users engage with their ads, watch their videos, play their games, etc. Tracking lets the advertiser learn what works and what doesn't work with their target audience.
Contact your client to determine exactly what they want to track in the creative. Some metrics are automatically tracked with rich media standard metrics.
Implement custom tracking by adding counters and timers to your creative. You can add up to 400 custom tracking elements (up to 100 exits and up to 300 counters and timers) to a single creative.