Documents to be carried when entering UK (or any other country) on a Visa

I note in many answers provided about 2nd visits or changed purpose, the respondents advise that you are likely to be interviewed by (British) immigration officer and should carry ALL the documents that you used to apply for your visa.

I do understand what that means but I do not understand WHY should I be carrying 6 months of payslips, bank statements, proof of property ownership etc.

Surely the output being an issued visa assessed all of that. If I can demonstrate on entry that I have means to support myself (currency in hand), means to exit (booked return ticket) and still have a reason to visit (in my case - sister turning 60 still resident in UK) then that should suffice.

I know the argument that a return plane ticket does not demonstrate intent to leave but I fail to see how 6 months of work history and financial records will demonstrate an intent to depart that was not already evaluated. It just seems unnecessary and I have not found the site that says "carry all the same documents for the border entry point". Perhaps someone can guide me to that.