Tags can be used to create very specific groups to send your promo campaigns to.
For example, some labels will use tags to tag by country, genre, media, radio, a-list, b-list etc.
When they go to create a campaign, they may choose to send to a specific country, targetting DJs of a specific genre, who work in radio.
On the Tag filter, you would select Australia + Techno + Radio.
All DJs with these three tags would then be filtered in your database.