I guess separation of concerns? One is for the system, one is for display?

A unique Boolean is easier to query than sorting and limiting?

Maybe you don't ever need the ability to display a history, but you still want the history (instead of just making the transaction_id unique constrained)... so you just drop the sort_order column?

I understand what you are saying.
Maybe that's just how they went about it. Originally planning on using created_at to do the ordering, then later realising that a customisable sort_order is required so they can make things make sense without having to fudge timestamps.