Progress you can see, not just track
A pile of cards doesn't tell you if you're on pace. A burndown chart does: it turns the cards in a milestone into a trend line of work remaining, so you can see at a glance whether it's shrinking fast enough to hit the target date. It updates itself as cards close, so there's no spreadsheet to keep in sync with the board by hand.
Built from the board you already have
Milestones aren't a separate project-management view bolted on top. They're made of the same cards, columns, and due dates already on your kanban board — group the cards that make up a launch or a client deliverable into a milestone, and the burndown chart sits right there in the same project as the notes, chat, and files behind that work.