This is the part most first-time visitors do not expect. An Uber cannot simply stop outside the arrivals doors. At Heathrow, app-based rides are directed to designated pickup areas for each terminal, which generally means walking from arrivals to a marked zone, often in or beside a short-stay car park.
That walk is manageable with a small case. With a family, a luggage trolley and a tired child it is noticeably harder, and at busy times you may be matched with a driver who is still some minutes away from the zone. A pre-booked driver, by contrast, can meet you inside arrivals with a name board if you choose meet & greet, or give you an exact level and bay before you walk out.