Default values in ad customizers allow you to provide an alternative value for each piece of customized text in your ads. So even if you are not using a data feed, your ads will always show.
For example, let's say you're running a campaign to advertise your candy store and you have an ad group that promotes your chocolate candy products. The snippet of code that you'll insert in your ad text might look like the following:
{=ProductFeed.CandyType:Chocolate} Doing this means Chocolate will show when the candy type can't be inserted in your ad.
Default values are optional. However, if an ad contains more than one ad customizer, default values must be provided for each customizer.