Let's say to get a list of leads you want to import, but you consider that the quality of this list is lower than your own data, so you would like to avoid overriding your data in Marketo, in the case where some of the leads you are importing are already in your DB. In this case, you can block some of the leads to be updated (if they have a value) and choose the "Untrusted" import mode in the import dialog.
This is for you to protect your existing precious data.
Let's say that your existing data is highly trusted, you know your email addresses, addresses and names are correct. You obtain a list (might be a purchased list) therefore you are not sure of the quality of this data, so you wouldn't want to overwrite your existing records. What you would do is to upload that new list as 'untrusted' and block fields from being updated from 'untrusted sources'.
This is how you can protect your existing data, you upload the new list but your fields are not modified by new data.