The other answers are good - For my experience specifically it was at a very very small agency, so the client chose us specifically for this purpose and knowing

The other answers are good - For my experience specifically it was at a very very small agency, so the client chose us specifically for this purpose and knowingly agreed to it. The first few months aren't just us working silently. It was always a very intensive process, I'd be on the phone with the client a few times a day to work through issues. They also get valuable tidbits of data early on. Most clients had ideas about how they expected data to look (70% of my revenue comes from product x) but often times the reality is different and they see immediate value. Clients sign up for a contract that auto-renews every three months and they have to give 30 days notice to terminate. At a small agency you have the luxury of denying clients all the time too that you don't want to work with.