Campaign tracking allows you to gather insight into which online marketing campaigns are generating the best leads and providing you with the most returned value. Tracking numbers that have an associated Campaign Tag are "sticky", meaning, the tracking number that the user sees as they navigate various pages of your website will continue to be the tracking number of the lead campaign, now and in the future.
For example, let's say you've just created a marketing campaign on Facebook for the winter season that targets a specific neighborhood in your city. A user clicks on your ad in Facebook and is directed to a landing page you've created about a special winter discount you're offering this season. The user wants to consider other options before making a decision. The next day, the user decides they would like to take advantage of your special winter offer and visits your website directly to find your contact information. The tracking numbers that the user sees on your website will be the tracking number associated with the marketing campaign you created on Facebook.
Because campaign tagged tracking numbers are "sticky", a user will continue seeing the tracking number that originally generated the lead. You get an end-to-end picture of a user's journey as well as the effective conversion rate of your marketing campaigns.