clickbank tracking ID

i am new to clickbank.. when it ask for tracking ID (optional) for my hop link..

1. what does the tracking ID do?

2. how do i benefit from it?

3. how do i get one?

Tracking IDs are used to distinguish between multiple traffic streams.

For example, if I have media buying traffic, email traffic, and SEO traffic, it can tell me which sales are coming from which traffic sources.