"Retain items for a specific Period" Vs "Only Delete items when they each a certain age"

The second option does not guarantee that the item will be preserved, the first one does. Think of the second option as "clean up". Whereas the first one guarantees that the item will be available at the end of the retention period, even if users try to delete it.

Read about the different retention settings here: https://learn.microsoft.com/en-us/purview/retention-settings#settings-for-retaining-and-deleting-content