Lead scoring is a feature to rank the contacts based on their activity and send appropriate campaign to appropriate group of contacts.
The ranking mechanism is to assign a score to each contacts based on their individual activities. The activities are Email Open, Link Click, Form Submit, Join List, Unsubscribe and so on
Some use case could be:
Identify contacts who are very active for last 3 weeks. (Means lead score threshold has changed to upward)
Identify contacts who were very active before 4 weeks but now quiet (means lead score threshold has changed to downwards)
Identify contacts who are active in last 2 weeks and part of a specific segment (segment based scoring) and we can build up such criteria based on requirement.
Once the contacts who fulfill these criteria are found, it is easy to trigger anniversary, campaign targeting these contacts