Advice not to obtain medical evidence specifically for UC50

I have just been reading UC 50 form and realised how strongly it emphasises that you should not obtain medical evidence specifically for the assessment. What is the reason for this? Is it just so claimants don’t feel a pressure to spend money?  For instance a focused report by a CPN could save a lot of time and it would certainly be needed at a MR or appeal .

Regardless of what it says, I will always attempt to get free medical evidence for the purpose of the ESA50 or UC50. In the past, this has largely been through CPN letters.