Frank prefers to use the private browsing mode on his browser because he knows that

this blocks advertisers from tracking his activity when he visits their Web sites

government agencies cannot monitor his browsing activities if he is suspected of cybercrime

this will prevent his sister, who sometimes uses his laptop, from accessing a list of the Web sites he visits

only private browsing ensures that the passwords he saves to his local device will not be used by a thief who steals his laptop