Your real estate agent is not a mind reader. If your real estate agent has no idea as to the amount you are approved by a mortgage lender to purchase a house, they would not be able to show you houses you are able to purchase.
It is not necessary to provide your real estate agent with a copy of your pre-approval letter, however, you should inform the real estate agent of the amount you were pre-approved for or show the your pre-approval letter.
What you have heard from the shade tree real estate agents and mortgage lenders, should be thrown out the window with the dirty water. The individuals that are giving you this erroneous information have no idea as to what they are speaking.
The real estate agent you select to show you houses, would not show you houses without being pre-approved. They do not want to waste their and your time showing you houses valued at $500k when you are only pre-approved for $250k to purchase a house.
I hope this has been of some benefit to you, good luck.