The mini-timeline allows, as the Summary does, navigation of a session. Its main advantage, though, is that it stays visible even when in Mixer mode.
The range of time covered by the mini-timeline is set by right clicking the timeline, and choosing a time span from 30 seconds up to 20 minutes.
The mini-timeline also shows all markers (start, end and any user defined ones). Clicking a marker jumps to that point on the timeline, allowing for quick access to key timings in the session.
While hovering with the mouse over the mini-timeline: