A fixed fare is easiest to compare against a metered taxi or an on-demand app once you separate the two things that make those prices hard to predict: the meter running longer in traffic, and demand-based surge pricing at busy times. With a fixed fare, both of those are already accounted for in the quote — a slower route on the day costs you nothing extra, and there is no surge multiplier applied at a busy hour or during bad weather. That does not automatically mean fixed-fare pricing is cheaper on every single trip, but it does mean the price you are quoted is the price on the invoice, with nothing added afterwards.
Where a fixed fare tends to show its value most clearly is exactly the kind of journey where the alternatives are least predictable: an early-morning airport run, a late-night arrival, a peak-hour city journey, or a booking made during a period of high demand such as New Year's Eve or a major event. Comparing like for like is straightforward: get an instant quote for your specific journey on the booking page, and you will see the exact figure before committing to anything.