because, when I do it or get it done by a trusted mechanic, I know what the state of things are. If the Previous Owner did the service, who knows what didn't get done or what the mechanic found but they kept out of the paperwork.
My PPI showed, for example, the need for brakes and a tire wearing unevenly. So I took that into account in my offer for the car and the seller knew what the PPI found too so they were not surprised by my lowering my offer. I had the work done and now have a car with new brakes, brake fluid, alignment and tires. So no worries for another 20k miles. I even had the 60k service done at 43k just to do some of the things I thought should be done based on time and not mileage.
Perhaps as a result, my Boxster has been extraordinarily reliable.
http://mike.focke.googlepages.com/mikesporscheboxsterwebpages