Why use a paintwork cleanser?
 

Paintwork cleansers are typically used to clean the paint well, thus promoting a better bond between the paint and your LSP. This in turn will improve the durability of your LSP. I have a Ford Focus in my shop that was in for a wash and wax, but after washing the paint still looked a little dull. Using my PC and a white pad, I applied some Werkstat Prime Strong. It was applied at speed 3, one panel at a time and then wiped down. For illustrative purposes, I used one pad for the entire car. Usually I would use 2 or 3 if the paint was this dirty.

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Why not clay/polish the car instead?

Because the customer was cheap and paid for a wash and wax. :troll:

Would have been my suggestion as well given the swirls but that would have added 4-5 hours to the job.

BINGO! The paint was too dirty to just seal and going over it with the paintwork cleanser only took 20 minutes and made a huge difference.

That's why I always have my trusty DG501 handy and a dedicate white pad for a paint cleansing job.

Which works better then? Cleaner or clay bar? I would suppose its faster with the cleaner though.

They are two totally different products. The clay is designed to remove bonded contaminants from the paint, not to actually clean, or "prep" the paint prior to LSP application. The Paintwork cleanser on the other hand, is designed to clean the paint of any oils, dirt etc that could hinder the LSP from bonding properly. IN a perfect work, wash, clay, paintwork cleanser, LSP. Client on a budget, skip the clay.