starts with / doesn't start with will allow you to use individual mapping methods for individual products or product groups.
You can assign some values to products depending on what the value starts with.
This option can be used if you want your Store Name to be the brand of some listings. If the source's title attributes start with the brand of the item (e.g. Nike) you can set it to Nike and set it to your Store Name if the value doesn't start with Nike.
It is always recommended to have a Fallback rule for products that are not matched by the condition, just to make sure that there are no empty values in your output feed.