The need for a stock ROM arises if the phone has already departed from the Samsung upgrade path.
Edited for clarity. In the case of a phone with the manufacturer’s implementation of Android intact, simply check the latest upgrade has been applied, restart and you are good to go.
Where you are not certain of history, perhaps check the Download mode (Odin mode) screen. “Warranty Void = 0 (or 0x0)” means Samsung registered no successful attempt on this device to customise. The wording “CURRENT BINARY: Samsung Official” is also shown.
If a stock ROM is required there are links here.
Bon chance 