Retention is not the epilogue to orthodontic treatment. It is the part that determines whether the result you paid for is still there in ten years.
Teeth are held in bone by fibres that remember where they used to be, and those fibres pull for a long time after the braces come off. On top of that, teeth drift very gradually throughout life whether or not you ever had orthodontic treatment. A retainer is what stops both.
We fit retainers the day the braces come off: no gap, no waiting for a lab. Fixed retainers are bonded behind the front teeth and do their job without you thinking about it; removable retainers are worn full-time at first and then at night indefinitely. Most patients end up with one of each.