Milestones are used to track important events in the customer journey e.g. lifecycle stage change, QBR/EBR, renewal, etc. Milestone are created both manually and in a lot of cases, rules are configured to create milestones after an event happens e.g. when EBR CTA is closed successfully, create a milestone.
The plan is to integrate milestones with timeline, since in timeline all important events/touches of the customer are recorded. Milestones will become an activity type.