Why would you want to purge a well with a pump that cannot be used to sample causing you to use a bailer after purging with the pump?

The only reason I know of would be a regulatory requirement. A good example of this is the Ivan County landfill – their regulatory agency specifically required that the samples be collected with a bailer and not with any type of pump. Although, later when the site switched over to dedicated water pumps they were allowed to take their samples with those. But, under the guidelines of the particular regulatory agency they had at the time, the open-phase electric submersible pumps they were using were not acceptable for groundwater sampling.  When a site uses a suction lift type of pump such as trash pumps or contractor pumps that are centrifugal pumps with an open phase, and pump at a pretty high rate they totally aerate the samples, so I can see the benefit of not using a pump like that to take samples.