The Stats tab shows your last 90 days as a grid, one square per day, filled in for days you completed a quest.
What it's for. A streak number tells you about right now. The board tells you about the shape of the last three months, which is more complete and usually more encouraging. Most people's boards have gaps and a clear upward trend, and seeing that is better than seeing a 4.
Milestones sit alongside it, marking runs worth noticing.
The board rolls, so it always shows the last 90 days rather than resetting.