There are two perspectives of your data lineage: lineage - where is your data flowing from, and impact analysis - what's on the collision course.
Sometimes, finding the source of the data can be a serious quest.
When you are reverse engineering a system it's good to start with something simple. That will help you see the big picture. Then you can get into more details.
Impact analysis is the process of determining what a change can break. Knowing that can make you feel powerful or terrified.
Understanding of source of the data can give you more confidence in the decisions made based on it.