A family of Microsoft relational database management systems designed for ease of use.
That doesn't make progress lines valid! My understanding is that progress lines were created purely because it fits the "unique " way they track in Japan.
Critical Path is important. The more time critical the project the more important CP is. You can't get accurate critical path if you have incomplete work in the past and some of your future tasks are already complete. You can't filter for tasks this week, you can't do a what if scenario for a new change request when your schedule can't accurately say what you still have to do when in the future (with incomplete work in the past).
Progress lines are wrong in so many ways and as Trevor said, many clients (and managers) have no idea what they should be asking for.
Use baselines, not progress lines to communicate progress.
If your client has asked for them I would demonstrate what should be done and how baselines provide much more relevant data. I get them to show me what happened since last week and since the last steering committee meeting 4 weeks ago and more.
Obviously if progress lines are the bees knees for them go with it, but if you want to control your project, use baselines as well!