All publishers are not equal on price. The price you are willing to pay for advertising inventory on publisher A is likely to vary from that of publisher B, and this is influenced by a number of factors including performance and perception. There the same principle applied to keyword bidding in the early days of PPC.
Publisher level bidding enables advertisers to set bid prices per publisher via the optimization report on the Smadex platform. The optimization reporting tool gives advertisers a powerful interface to interrogate campaign activity at the most granular level, and from there determine publisher level pricing options. However, with so many publishers, planning bidding strategies does take time to analyse in order to determine the most effective bids.
The advantage that publisher level bidding provides is increased bid level granularity, and that can translate into improved results and therefore likely to drive higher auction win rates. Also, by neglecting publisher level bidding, you could be overpaying for publisher inventory by using one price for hundreds of thousands of possible publisher options.
For performance campaigns, the increased visibility and control enables advertisers to set publisher level bid strategies and increase win rates at better prices therefore contributing to an overall better performance at lower cost. As an example, there may be one publisher that converts extremely well but has a very high floor price, so instead of raising your bid for all, you can set an appropriate bid for that publisher and channel your budgets into the places that work.
For brand campaigns, the concept applies, but enabling bidding at publisher level, advertisers can be a lot more efficient and effective with their advertising dollars.
In summary, publisher level bidding provides a more efficient buying strategy but requires analysis and time to determine optimum pricing levels to drives the best performance.