track links update times and update services whts the difference

I dont get the point after reading codex on whts the difference between track link update times option and update services options. I understand that if you have put update services (pingomatic is put by default) every time you post or update a post services aimed by pingomatic will be told you put new content on your site. But dont understand wht track links update times do, it sound to be the same and is disabled by default.

thanks for reading