Bookmarking ads is best used for saving individual ad creatives. Bookmarking ads will place them in a folder, allowing you to easily organize creatives, as well as continue to gather placement and landing page information on them as they're continuously updated.
Bookmarking advertisers allows you to quickly gather all the ads, placements, LPs, and other assets for multiple advertisers. This feature is best used by creating folders to bookmark multiple similar advertisers. After bookmarking multiple advertisers in a similar niche, the overview tab will start to develop a very clear narrative on how other advertisers approach buying media in your target category.
In the example below, multiple 'medicare' advertisers were bookmarked, presenting a clear snapshot on the best approach to start buying display media for my next medicare related campaign.