Here are some:
#1 – you will have additional time to safely review the sonar data without having to worry about running into/over something or someone running into/over you while on the water. This added time to study the sonar data may allow you to see details and structures that may hold fish that you did not see while on the water. Waypoints can still be marked and you can still use almost all of the menus (like sonar zoom).
#2 (if viewing on a computer) – the units record sonar data at a higher definition than can be shown on the units themselves. So reviewing the sonar data on a larger computer screen may allow you to see details you missed before.
#3 – memory (yours, not the unit’s or SD card’s): when reviewing the sonar data you can write/type notes for different areas or waypoints that may help you successfully fish them later. Maybe its just me but after a day on the boat I can’t remember which fish we caught where unless I can remember what the bottom looked like.
Reviewing sonar recordings for Si sonar makes even more sense due to the amount of data that is present, but it can also help with 2D only sonar units as well.