Default pins appear on Data Entry pages.
They can be used to set the defaults for specific fields so that each time you access that page these settings are defaulted into the selections for you. You do not have to then constantly choose these settings again and only change them if you need to.
They are useful for rapid data entry where you constantly enter data of the same type and want to have the page load with the defaults already set for you.
All such defaults are personal to each User.