Add display conditions
Imagine you need a repeating content field with two or more different subtypes. For example: a repeating linklist-field with downloads (consisting of a filefield and a label) and links (consisting of a linkfield and a label).
At the moment you can create only one subtype.
With displayconditions exists a type-chooser. You can assign as lot of fields as you need for each type. Also a multiple usage of fields in more than one type is possible.
Perhaps you know this feature of TYPO3 from the page-properties when you switch the page-type. Than the form reloads and shows different fields.
Display conditions should be manageable in the Mask-wizard.