The upsides of using Garrison Mode
The good thing about Garrison Mode is that you can save your units after they are weakened. If you have enough money, you can get units when your units are still in Garrison mode, and then you can attack the enemy after you have gotten enough units.
The downsides of using Garrison Mode
There are downsides of using Garrison Mode, however. If you are retreating from an enemy who is a Spearton or a Swordwrath, you would be doing the enemy some good by letting them destroy your statue without being bothered (only if you have Archidons in your base.)