Most tax work is commissioned in February and delivered in April. That sequence guarantees that the largest decisions are reviewed after they can no longer be changed.
A standing relationship changes the order of operations. When your accountant knows a sale is coming, a move is planned, or a child is joining the business, the analysis can happen while there is still leverage.
It also changes what you receive. Instead of a return and a bill, you get a short list of things worth deciding this quarter — and the reasoning behind each one.
This is the older way of practicing, and it still works better than any software that promises to replace it.