As illustrated below, when an Advertiser traffics an Ad Tag to a Publisher, the tag is uploaded to the Publisher's Ad Server. When a page that includes the code for the uploaded tag is loaded on the Publisher's site, a 'call' is made to the Advertiser's Ad Server, which then returns a call that places the relevant content into the ad tag.
But if I had to give my #1 benefit it would be this: Serving content within an Ad Tag allows the Advertiser to rotate as many content pieces as they like, whenever they like. Since all of the action is happening within their Ad Server this allows for quick and easy performance optimizations.
And the #1 concern associated with running Ad Tags? The Advertiser can rotate whatever they like, whenever they like. Since updating content within tags (a.k.a. making changes on the back end) does not require trafficking anything new, Publishers can be caught unawares in the hands of unscrupulous advertisers, so random spot checks are never a bad idea.