Dynamic attributes are meant for supporting remarketing advertising. That is, setting information when a user visits your site so that you can later target them with ads when they visit other sites.
Remarketing is an effective advertising strategy. It reminds people of your site so they come back. You may want to advertise to them that they can purchase an item that they viewed or added to their cart but never purchased. Basically any time a user visited your site but didn't "convert", you might want to get your message in front of them later that they can still come back and complete your desired action on your site.
You can also show them ads for items that are similar or may complement an item they already bought.
Only dynamic attributes in Google Analytics support this connection to retargeting ads. Custom dimensions and metrics can't be used to target ads on other sites.
You want to set dynamic attributes for:
- what people viewed
- what they added to their cart
- if they purchased
- the category of item
- where the user or item is located
- the value of the item
- relevant additional details that they searched for such as dates of travel
These details will allow your advertising team figure out which users to retarget and which ads would most entice them to come back.