If you come across several tracks pointing towards your own kingdom you could assume that it is an enemy campaign, this way you can track them down and either assemble an army to beat them, or fight on the walls of the castle/city they are trying to take.
Its also usefull when hunting bandits, when you are in an area where you know there should be sea raiders and you see a track showing a partysize of less then 50 and more then 20 you could be pretty sure that this is a sea raider party, so you track it down with ease.
Tracking save time, but it is not a must to have.
I get Deshavi (or whatever her name is) since I always pick her, she have tracking 2, I don't feel that I need better tracking then that.
Of course, tracks give more information the higher skill you have. I have never had tracking at 10 but I guess the tracks would last longer and be much more informative showing what faction it is, and perhaps even what lord it it.
But then again, I might be wrong since I have never tried it out myself : )