The default Salesforce OpportunityHistory field is not set up to answer the following questions out of the box:
How much net new pipeline is being created over time?
How is each person impacting pipeline changes over time?
How consistently are we moving #stages forward over time
How consistently are we moving $ forward over time?
How does my win rate change by each stage and how is it changing over time?
What is my average time in stage and how is it changing over time?
OpportunityChange is a custom object that captures the data you need to answer all of these questions from a single report.
Why does Truly Track OpportunityChanges?
A key component of automated pipeline inspection is understanding how activity is impacting opportunity progression. OpportunityChanges are used to model Opportunity level custom metrics that are the foundation of automated pipeline inspection.
You can read more about Calculated Opportunity Metrics and Opportunity Trackers.