Think of a product offering, giving an interface to manage advertising on several search engines. Each search engine has its own development programme. You could distribute software that interacts directly with each serach engine. If you do, you need to issue an update each time an interface changes. Consequence is lots of updates. In practice, clients tend to hate more than a couple of new software revisions per year... so you may have acceptance problems.

OTOH, issue software that deals with a server that you have. Your server interacts with each SE. So long as the abstraction model offered by your service is sufficiently general, changes to paid search programme API will not significantly affect your servers interface - so client software remains more stable. You make one set of changes and control the rollout.

Example: Atlas. This works with a UI that submits changes to a hosted service managed by Atlas. That then runs the changes you request and does the reporting. Disadvantages? If the abstraction model is insufficiently complex, you can't deal with, for example, multiple adverts or the difference between broad and exact match versions of the same keyword.

Cheers, JeremyC.